--- layout: "docs" page_title: "Upgrading Specific Versions" sidebar_current: "docs-upgrading-specific" description: |- Specific versions of Consul may have additional information about the upgrade process beyond the standard flow. --- # Upgrading Specific Versions The [upgrading page](/docs/upgrading.html) covers the details of doing a standard upgrade. However, specific versions of Consul may have more details provided for their upgrades as a result of new features or changed behavior. This page is used to document those details separately from the standard upgrade flow. ## Consul 1.0.1 #### Carefully Check and Remove Stale Servers During Rolling Upgrades Consul 1.0 (and earlier versions of Consul when running with [Raft protocol 3](/docs/agent/options.html#_raft_protocol) had an issue where performing rolling updates of Consul servers could result in an outage from old servers remaining in the cluster. [Autopilot](/docs/guides/autopilot.html) would normally remove old servers when new ones come online, but it was also waiting to promote servers to voters in pairs to maintain an odd quorum size. The pairwise promotion feature was removed so that servers become voters as soon as they are stable, allowing Autopilot to remove old servers in a safer way. When upgrading from Consul 1.0, you may need to manually [force-leave](/docs/commands/force-leave.html) old servers as part of a rolling update to Consul 1.0.1. ## Consul 1.0 Consul 1.0 has several important breaking changes that are documented here. Please be sure to read over all the details here before upgrading. #### Raft Protocol Now Defaults to 3 The [`-raft-protocol`](/docs/agent/options.html#_raft_protocol) default has been changed from 2 to 3, enabling all [Autopilot](/docs/guides/autopilot.html) features by default. Raft protocol version 3 requires Consul running 0.8.0 or newer on all servers in order to work, so if you are upgrading with older servers in a cluster then you will need to set this back to 2 in order to upgrade. See [Raft Protocol Version Compatibility](/docs/upgrade-specific.html#raft-protocol-version-compatibility) for more details. Also the format of `peers.json` used for outage recovery is different when running with the lastest Raft protocol. See [Manual Recovery Using peers.json](/docs/guides/outage.html#manual-recovery-using-peers-json) for a description of the required format. Please note that the Raft protocol is different from Consul's internal protocol as described on the [Protocol Compatibility Promise](/docs/compatibility.html) page, and as is shown in commands like `consul members` and `consul version`. To see the version of the Raft protocol in use on each server, use the `consul operator raft list-peers` command. The easiest way to upgrade servers is to have each server leave the cluster, upgrade its Consul version, and then add it back. Make sure the new server joins successfully and that the cluster is stable before rolling the upgrade forward to the next server. It's also possible to stand up a new set of servers, and then slowly stand down each of the older servers in a similar fashion. When using Raft protocol version 3, servers are identified by their [`-node-id`](/docs/agent/options.html#_node_id) instead of their IP address when Consul makes changes to its internal Raft quorum configuration. This means that once a cluster has been upgraded with servers all running Raft protocol version 3, it will no longer allow servers running any older Raft protocol versions to be added. If running a single Consul server, restarting it in-place will result in that server not being able to elect itself as a leader. To avoid this, either set the Raft protocol back to 2, or use [Manual Recovery Using peers.json](/docs/guides/outage.html#manual-recovery-using-peers-json) to map the server to its node ID in the Raft quorum configuration. #### Config Files Require an Extension As part of supporting the [HCL](https://github.com/hashicorp/hcl#syntax) format for Consul's config files, an `.hcl` or `.json` extension is required for all config files loaded by Consul, even when using the [`-config-file`](/docs/agent/options.html#_config_file) argument to specify a file directly. #### Deprecated Options Have Been Removed All of Consul's previously deprecated command line flags and config options have been removed, so these will need to be mapped to their equivalents before upgrading. Here's the complete list of removed options and their equivalents: | Removed Option | Equivalent | | -------------- | ---------- | | `-atlas` | None, Atlas is no longer supported. | | `-atlas-token`| None, Atlas is no longer supported. | | `-atlas-join` | None, Atlas is no longer supported. | | `-atlas-endpoint` | None, Atlas is no longer supported. | | `-dc` | [`-datacenter`](/docs/agent/options.html#_datacenter) | | `-retry-join-azure-tag-name` | [`-retry-join`](/docs/agent/options.html#microsoft-azure) | | `-retry-join-azure-tag-value` | [`-retry-join`](/docs/agent/options.html#microsoft-azure) | | `-retry-join-ec2-region` | [`-retry-join`](/docs/agent/options.html#amazon-ec2) | | `-retry-join-ec2-tag-key` | [`-retry-join`](/docs/agent/options.html#amazon-ec2) | | `-retry-join-ec2-tag-value` | [`-retry-join`](/docs/agent/options.html#amazon-ec2) | | `-retry-join-gce-credentials-file` | [`-retry-join`](/docs/agent/options.html#google-compute-engine) | | `-retry-join-gce-project-name` | [`-retry-join`](/docs/agent/options.html#google-compute-engine) | | `-retry-join-gce-tag-name` | [`-retry-join`](/docs/agent/options.html#google-compute-engine) | | `-retry-join-gce-zone-pattern` | [`-retry-join`](/docs/agent/options.html#google-compute-engine) | | `addresses.rpc` | None, the RPC server for CLI commands is no longer supported. | | `advertise_addrs` | [`ports`](/docs/agent/options.html#ports) with [`advertise_addr`](https://www.consul/io/docs/agent/options.html#advertise_addr) and/or [`advertise_addr_wan`](/docs/agent/options.html#advertise_addr_wan) | | `atlas_infrastructure` | None, Atlas is no longer supported. | | `atlas_token` | None, Atlas is no longer supported. | | `atlas_acl_token` | None, Atlas is no longer supported. | | `atlas_join` | None, Atlas is no longer supported. | | `atlas_endpoint` | None, Atlas is no longer supported. | | `dogstatsd_addr` | [`telemetry.dogstatsd_addr`](/docs/agent/options.html#telemetry-dogstatsd_addr) | | `dogstatsd_tags` | [`telemetry.dogstatsd_tags`](/docs/agent/options.html#telemetry-dogstatsd_tags) | | `http_api_response_headers` | [`http_config.response_headers`](/docs/agent/options.html#response_headers) | | `ports.rpc` | None, the RPC server for CLI commands is no longer supported. | | `recursor` | [`recursors`](https://github.com/hashicorp/consul/blob/master/website/source/docs/agent/options.html.md#recursors) | | `retry_join_azure` | [`-retry-join`](/docs/agent/options.html#microsoft-azure) | | `retry_join_ec2` | [`-retry-join`](/docs/agent/options.html#amazon-ec2) | | `retry_join_gce` | [`-retry-join`](/docs/agent/options.html#google-compute-engine) | | `statsd_addr` | [`telemetry.statsd_address`](https://github.com/hashicorp/consul/blob/master/website/source/docs/agent/options.html.md#telemetry-statsd_address) | | `statsite_addr` | [`telemetry.statsite_address`](https://github.com/hashicorp/consul/blob/master/website/source/docs/agent/options.html.md#telemetry-statsite_address) | | `statsite_prefix` | [`telemetry.metrics_prefix`](/docs/agent/options.html#telemetry-metrics_prefix) | | `telemetry.statsite_prefix` | [`telemetry.metrics_prefix`](/docs/agent/options.html#telemetry-metrics_prefix) | | (service definitions) `serviceid` | [`service_id`](/docs/agent/services.html) | | (service definitions) `dockercontainerid` | [`docker_container_id`](/docs/agent/services.html) | | (service definitions) `tlsskipverify` | [`tls_skip_verify`](/docs/agent/services.html) | | (service definitions) `deregistercriticalserviceafter` | [`deregister_critical_service_after`](/docs/agent/services.html) | #### `statsite_prefix` Renamed to `metrics_prefix` Since the `statsite_prefix` configuration option applied to all telemetry providers, `statsite_prefix` was renamed to [`metrics_prefix`](/docs/agent/options.html#telemetry-metrics_prefix). Configuration files will need to be updated when upgrading to this version of Consul. #### `advertise_addrs` Removed This configuration option was removed since it was redundant with `advertise_addr` and `advertise_addr_wan` in combination with `ports` and also wrongly stated that you could configure both host and port. #### Escaping Behavior Changed for go-discover Configs The format for [`-retry-join`](/docs/agent/options.html#retry-join) and [`-retry-join-wan`](/docs/agent/options.html#retry-join-wan) values that use [go-discover](https://github.com/hashicorp/go-discover) cloud auto joining has changed. Values in `key=val` sequences must no longer be URL encoded and can be provided as literals as long as they do not contain spaces, backslashes `\` or double quotes `"`. If values contain these characters then use double quotes as in `"some key"="some value"`. Special characters within a double quoted string can be escaped with a backslash `\`. #### HTTP Verbs are Enforced in Many HTTP APIs Many endpoints in the HTTP API that previously took any HTTP verb now check for specific HTTP verbs and enforce them. This may break clients relying on the old behavior. Here's the complete list of updated endpoints and required HTTP verbs: | Endpoint | Required HTTP Verb | | -------- | ------------------ | | /v1/acl/info | GET | | /v1/acl/list | GET | | /v1/acl/replication | GET | | /v1/agent/check/deregister | PUT | | /v1/agent/check/fail | PUT | | /v1/agent/check/pass | PUT | | /v1/agent/check/register | PUT | | /v1/agent/check/warn | PUT | | /v1/agent/checks | GET | | /v1/agent/force-leave | PUT | | /v1/agent/join | PUT | | /v1/agent/members | GET | | /v1/agent/metrics | GET | | /v1/agent/self | GET | | /v1/agent/service/register | PUT | | /v1/agent/service/deregister | PUT | | /v1/agent/services | GET | | /v1/catalog/datacenters | GET | | /v1/catalog/deregister | PUT | | /v1/catalog/node | GET | | /v1/catalog/nodes | GET | | /v1/catalog/register | PUT | | /v1/catalog/service | GET | | /v1/catalog/services | GET | | /v1/coordinate/datacenters | GET | | /v1/coordinate/nodes | GET | | /v1/health/checks | GET | | /v1/health/node | GET | | /v1/health/service | GET | | /v1/health/state | GET | | /v1/internal/ui/node | GET | | /v1/internal/ui/nodes | GET | | /v1/internal/ui/services | GET | | /v1/session/info | GET | | /v1/session/list | GET | | /v1/session/node | GET | | /v1/status/leader | GET | | /v1/status/peers | GET | | /v1/operator/area/:uuid/members | GET | | /v1/operator/area/:uuid/join | PUT | #### Unauthorized KV Requests Return 403 When ACLs are enabled, reading a key with an unauthorized token returns a 403. This previously returned a 404 response. #### Config Section of Agent Self Endpoint has Changed The /v1/agent/self endpoint's `Config` section has often been in flux as it was directly returning one of Consul's internal data structures. This configuration structure has been moved under `DebugConfig`, and is documents as for debugging use and subject to change, and a small set of elements of `Config` have been maintained and documented. See [Read Configuration](/api/agent.html#read-configuration) endpoint documentation for details. #### Deprecated `configtest` Command Removed The `configtest` command was deprecated and has been superseded by the `validate` command. #### Undocumented Flags in `validate` Command Removed The `validate` command supported the `-config-file` and `-config-dir` command line flags but did not document them. This support has been removed since the flags are not required. #### Metric Names Updated Metric names no longer start with `consul.consul`. To help with transitioning dashboards and other metric consumers, the field `enable_deprecated_names` has been added to the telemetry section of the config, which will enable metrics with the old naming scheme to be sent alongside the new ones. The following prefixes were affected: | Prefix | | ------ | | consul.consul.acl | | consul.consul.autopilot | | consul.consul.catalog | | consul.consul.fsm | | consul.consul.health | | consul.consul.http | | consul.consul.kvs | | consul.consul.leader | | consul.consul.prepared-query | | consul.consul.rpc | | consul.consul.session | | consul.consul.session_ttl | | consul.consul.txn | #### Checks Validated On Agent Startup Consul agents now validate health check definitions in their configuration and will fail at startup if any checks are invalid. In previous versions of Consul, invalid health checks would get skipped. ## Consul 0.9.0 #### Script Checks Are Now Opt-In A new [`enable_script_checks`](/docs/agent/options.html#_enable_script_checks) configuration option was added, and defaults to `false`, meaning that in order to allow an agent to run health checks that execute scripts, this will need to be configured and set to `true`. This provides a safer out-of-the-box configuration for Consul where operators must opt-in to allow script-based health checks. If your cluster uses script health checks please be sure to set this to `true` as part of upgrading agents. If this is set to `true`, you should also enable [ACLs](/docs/guides/acl.html) to provide control over which users are allowed to register health checks that could potentially execute scripts on the agent machines. #### Web UI Is No Longer Released Separately Consul releases will no longer include a `web_ui.zip` file with the compiled web assets. These have been built in to the Consul binary since the 0.7.x series and can be enabled with the [`-ui`](/docs/agent/options.html#_ui) configuration option. These built-in web assets have always been identical to the contents of the `web_ui.zip` file for each release. The [`-ui-dir`](/docs/agent/options.html#_ui_dir) option is still available for hosting customized versions of the web assets, but the vast majority of Consul users can just use the built in web assets. ## Consul 0.8.0 #### Upgrade Current Cluster Leader Last We identified a potential issue with Consul 0.8 that requires the current cluster leader to be upgraded last when updating multiple servers. Please see [this issue](https://github.com/hashicorp/consul/issues/2889) for more details. #### Command-Line Interface RPC Deprecation The RPC client interface has been removed. All CLI commands that used RPC and the `-rpc-addr` flag to communicate with Consul have been converted to use the HTTP API and the appropriate flags for it, and the `rpc` field has been removed from the port and address binding configs. You will need to remove these fields from your config files and update any scripts that passed a custom `-rpc-addr` to the following commands: * `force-leave` * `info` * `join` * `keyring` * `leave` * `members` * `monitor` * `reload` #### Version 8 ACLs Are Now Opt-Out The [`acl_enforce_version_8`](/docs/agent/options.html#acl_enforce_version_8) configuration now defaults to `true` to enable [full version 8 ACL support](/docs/guides/acl.html#version_8_acls) by default. If you are upgrading an existing cluster with ACLs enabled, you will need to set this to `false` during the upgrade on **both Consul agents and Consul servers**. Version 8 ACLs were also changed so that [`acl_datacenter`](/docs/agent/options.html#acl_datacenter) must be set on agents in order to enable the agent-side enforcement of ACLs. This makes for a smoother experience in clusters where ACLs aren't enabled at all, but where the agents would have to wait to contact a Consul server before learning that. #### Remote Exec Is Now Opt-In The default for [`disable_remote_exec`](/docs/agent/options.html#disable_remote_exec) was changed to "true", so now operators need to opt-in to having agents support running commands remotely via [`consul exec`](/docs/commands/exec.html). #### Raft Protocol Version Compatibility When upgrading to Consul 0.8.0 from a version lower than 0.7.0, users will need to set the [`-raft-protocol`](/docs/agent/options.html#_raft_protocol) option to 1 in order to maintain backwards compatibility with the old servers during the upgrade. After the servers have been migrated to version 0.8.0, `-raft-protocol` can be moved up to 2 and the servers restarted to match the default. The Raft protocol must be stepped up in this way; only adjacent version numbers are compatible (for example, version 1 cannot talk to version 3). Here is a table of the Raft Protocol versions supported by each Consul version:
Version Supported Raft Protocols
0.6 and earlier 0
0.7 1
0.8 1, 2, 3
In order to enable all [Autopilot](/docs/guides/autopilot.html) features, all servers in a Consul cluster must be running with Raft protocol version 3 or later. ## Consul 0.7.1 #### Child Process Reaping Child process reaping support has been removed, along with the `reap` configuration option. Reaping is also done via [dumb-init](https://github.com/Yelp/dumb-init) in the [Consul Docker image](https://github.com/hashicorp/docker-consul), so removing it from Consul itself simplifies the code and eases future maintenance for Consul. If you are running Consul as PID 1 in a container you will need to arrange for a wrapper process to reap child processes. #### DNS Resiliency Defaults The default for [`max_stale`](/docs/agent/options.html#max_stale) has been increased from 5 seconds to a near-indefinite threshold (10 years) to allow DNS queries to continue to be served in the event of a long outage with no leader. A new telemetry counter was added at `consul.dns.stale_queries` to track when agents serve DNS queries that are stale by more than 5 seconds. ## Consul 0.7 Consul version 0.7 is a very large release with many important changes. Changes to be aware of during an upgrade are categorized below. #### Performance Timing Defaults and Tuning Consul 0.7 now defaults the DNS configuration to allow for stale queries by defaulting [`allow_stale`](/docs/agent/options.html#allow_stale) to true for better utilization of available servers. If you want to retain the previous behavior, set the following configuration: ```javascript { "dns_config": { "allow_stale": false } } ``` Consul also 0.7 introduced support for tuning Raft performance using a new [performance configuration block](/docs/agent/options.html#performance). Also, the default Raft timing is set to a lower-performance mode suitable for [minimal Consul servers](/docs/guides/performance.html#minumum). To continue to use the high-performance settings that were the default prior to Consul 0.7 (recommended for production servers), add the following configuration to all Consul servers when upgrading: ```javascript { "performance": { "raft_multiplier": 1 } } ``` See the [Server Performance](/docs/guides/performance.html) guide for more details. #### Leave-Related Configuration Defaults The default behavior of [`leave_on_terminate`](/docs/agent/options.html#leave_on_terminate) and [`skip_leave_on_interrupt`](/docs/agent/options.html#skip_leave_on_interrupt) are now dependent on whether or not the agent is acting as a server or client: * For servers, `leave_on_terminate` defaults to "false" and `skip_leave_on_interrupt` defaults to "true". * For clients, `leave_on_terminate` defaults to "true" and `skip_leave_on_interrupt` defaults to "false". These defaults are designed to be safer for servers so that you must explicitly configure them to leave the cluster. This also results in a better experience for clients, especially in cloud environments where they may be created and destroyed often and users prefer not to wait for the 72 hour reap time for cleanup. #### Dropped Support for Protocol Version 1 Consul version 0.7 dropped support for protocol version 1, which means it is no longer compatible with versions of Consul prior to 0.3. You will need to upgrade all agents to a newer version of Consul before upgrading to Consul 0.7. #### Prepared Query Changes Consul version 0.7 adds a feature which allows prepared queries to store a [`Near` parameter](/api/query.html#near) in the query definition itself. This feature enables using the distance sorting features of prepared queries without explicitly providing the node to sort near in requests, but requires the agent servicing a request to send additional information about itself to the Consul servers when executing the prepared query. Agents prior to 0.7 do not send this information, which means they are unable to properly execute prepared queries configured with a `Near` parameter. Similarly, any server nodes prior to version 0.7 are unable to store the `Near` parameter, making them unable to properly serve requests for prepared queries using the feature. It is recommended that all agents be running version 0.7 prior to using this feature. #### WAN Address Translation in HTTP Endpoints Consul version 0.7 added support for translating WAN addresses in certain [HTTP endpoints](/docs/agent/options.html#translate_wan_addrs). The servers and the agents need to be running version 0.7 or later in order to use this feature. These translated addresses could break HTTP endpoint consumers that are expecting local addresses, so a new [`X-Consul-Translate-Addresses`](/api/index.html#translate_header) header was added to allow clients to detect if translation is enabled for HTTP responses. A "lan" tag was added to `TaggedAddresses` for clients that need the local address regardless of translation. #### Outage Recovery and `peers.json` Changes The `peers.json` file is no longer present by default and is only used when performing recovery. This file will be deleted after Consul starts and ingests the file. Consul 0.7 also uses a new, automatically-created raft/peers.info file to avoid ingesting the `peers.json` file on the first start after upgrading (the `peers.json` file is simply deleted on the first start after upgrading). Please be sure to review the [Outage Recovery Guide](/docs/guides/outage.html) before upgrading for more details. ## Consul 0.6.4 Consul 0.6.4 made some substantial changes to how ACLs work with prepared queries. Existing queries will execute with no changes, but there are important differences to understand about how prepared queries are managed before you upgrade. In particular, prepared queries with no `Name` defined will no longer require any ACL to manage them, and prepared queries with a `Name` defined are now governed by a new `query` ACL policy that will need to be configured after the upgrade. See the [ACL Guide](/docs/guides/acl.html#prepared_query_acls) for more details about the new behavior and how it compares to previous versions of Consul. ## Consul 0.6 Consul version 0.6 is a very large release with many enhancements and optimizations. Changes to be aware of during an upgrade are categorized below. #### Data Store Changes Consul changed the format used to store data on the server nodes in version 0.5 (see 0.5.1 notes below for details). Previously, Consul would automatically detect data directories using the old LMDB format, and convert them to the newer BoltDB format. This automatic upgrade has been removed for Consul 0.6, and instead a safeguard has been put in place which will prevent Consul from booting if the old directory format is detected. It is still possible to migrate from a 0.5.x version of Consul to 0.6+ using the [consul-migrate](https://github.com/hashicorp/consul-migrate) CLI utility. This is the same tool that was previously embedded into Consul. See the [releases](https://github.com/hashicorp/consul-migrate/releases) page for downloadable versions of the tool. Also, in this release Consul switched from LMDB to a fully in-memory database for the state store. Because LMDB is a disk-based backing store, it was able to store more data than could fit in RAM in some cases (though this is not a recommended configuration for Consul). If you have an extremely large data set that won't fit into RAM, you may encounter issues upgrading to Consul 0.6.0 and later. Consul should be provisioned with physical memory approximately 2X the data set size to allow for bursty allocations and subsequent garbage collection. #### ACL Enhancements Consul 0.6 introduces enhancements to the ACL system which may require special handling: * Service ACLs are enforced during service discovery (REST + DNS) Previously, service discovery was wide open, and any client could query information about any service without providing a token. Consul now requires read-level access at a minimum when ACLs are enabled to return service information over the REST or DNS interfaces. If clients depend on an open service discovery system, then the following should be added to all ACL tokens which require it: # Enable discovery of all services service "" { policy = "read" } When the DNS interface is queried, the agent's [`acl_token`](/docs/agent/options.html#acl_token) is used, so be sure that token has sufficient privileges to return the DNS records you expect to retrieve from it. * Event and keyring ACLs Similar to service discovery, the new event and keyring ACLs will block access to these operations if the `acl_default_policy` is set to `deny`. If clients depend on open access to these, then the following should be added to all ACL tokens which require them: event "" { policy = "write" } keyring = "write" Unfortunately, these are new ACLs for Consul 0.6, so they must be added after the upgrade is complete. #### Prepared Queries Prepared queries introduce a new Raft log entry type that isn't supported on older versions of Consul. It's important to not use the prepared query features of Consul until all servers in a cluster have been upgraded to version 0.6.0. #### Single Private IP Enforcement Consul will refuse to start if there are multiple private IPs available, so if this is the case you will need to configure Consul's advertise or bind addresses before upgrading. #### New Web UI File Layout The release .zip file for Consul's web UI no longer contains a `dist` sub-folder; everything has been moved up one level. If you have any automated scripts that expect the old layout you may need to update them. ## Consul 0.5.1 Consul version 0.5.1 uses a different backend store for persisting the Raft log. Because of this change, a data migration is necessary to move the log entries out of LMDB and into the newer backend, BoltDB. Consul version 0.5.1+ makes this transition seamless and easy. As a user, there are no special steps you need to take. When Consul starts, it checks for presence of the legacy LMDB data files, and migrates them automatically if any are found. You will see a log emitted when Raft data is migrated, like this: ``` ==> Successfully migrated raft data in 5.839642ms ``` This automatic upgrade will only exist in Consul 0.5.1+ and it will be removed starting with Consul 0.6.0+. It will still be possible to upgrade directly from pre-0.5.1 versions by using the consul-migrate utility, which is available on the [Consul Tools page](/downloads_tools.html). ## Consul 0.5 Consul version 0.5 adds two features that complicate the upgrade process: * ACL system includes service discovery and registration * Internal use of tombstones to fix behavior of blocking queries in certain edge cases. Users of the ACL system need to be aware that deploying Consul 0.5 will cause service registration to be enforced. This means if an agent attempts to register a service without proper privileges it will be denied. If the `acl_default_policy` is "allow" then clients will continue to work without an updated policy. If the policy is "deny", then all clients will begin to have their registration rejected causing issues. To avoid this situation, all the ACL policies should be updated to add something like this: # Enable all services to be registered service "" { policy = "write" } This will set the service policy to `write` level for all services. The blank service name is the catch-all value. A more specific service can also be specified: # Enable only the API service to be registered service "api" { policy = "write" } The ACL policy can be updated while running 0.4, and enforcement will being with the upgrade to 0.5. The policy updates will ensure the availability of the cluster. The second major change is the new internal command used for tombstones. The details of the change are not important, however to function the leader node will replicate a new command to its followers. Consul is designed defensively, and when a command that is not recognized is received, the server will panic. This is a purposeful design decision to avoid the possibility of data loss, inconsistencies, or security issues caused by future incompatibility. In practice, this means if a Consul 0.5 node is the leader, all of its followers must also be running 0.5. There are a number of ways to do this to ensure cluster availability: * Add new 0.5 nodes, then remove the old servers. This will add the new nodes as followers, and once the old servers are removed, one of the 0.5 nodes will become leader. * Upgrade the followers first, then the leader last. Using `consul info`, you can determine which nodes are followers. Do an in-place upgrade on them first, and finally upgrade the leader last. * Upgrade them in any order, but ensure all are done within 15 minutes. Even if the leader is upgraded to 0.5 first, as long as all of the followers are running 0.5 within 15 minutes there will be no issues. Finally, even if any of the methods above are not possible or the process fails for some reason, it is not fatal. The older version of the server will simply panic and stop. At that point, you can upgrade to the new version and restart the agent. There will be no data loss and the cluster will resume operations.