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Frequently Asked Questions
Q: Why is virtual memory usage high?
Consul makes use of LMDB internally for various data storage purposes. LMDB relies on using memory-mapping, a technique in which a sparse file is represented as a contiguous range of memory. Consul configures high limits for these file sizes and as a result relies on large chunks of virtual memory to be allocated. However, in practice, the limits are much larger than any realistic deployment of Consul would ever use, and the resident memory or physical memory used is much lower.
Q: What is Checkpoint? / Does Consul call home?
Consul makes use of a HashiCorp service called Checkpoint which is used to check for updates and critical security bulletins. Only anonymous information, which cannot be used to identify the user or host, is sent to Checkpoint . An anonymous ID is sent which helps de-duplicate warning messages. This anonymous ID can can be disabled. In fact, using the Checkpoint service is optional and can be disabled.
See disable_anonymous_signature
and disable_update_check.
Q: How does Atlas integration work?
Consul makes use of a HashiCorp service called SCADA (Supervisory Control And Data Acquisition). The SCADA system allows clients to maintain long-running connections to Atlas. Atlas can in turn provide auto-join facilities for Consul agents (supervisory control) and an integrated dashboard showing the health of all connected agents (data acquisition).
Standard ACLs can be applied to the SCADA connection, ensuring that Atlas is given only those privileges that make sense for your deployment.
Using the SCADA service is optional. SCADA is only enabled by opt-in.
See the Atlas integration guide for more details.
Q: Does Consul rely on UDP Broadcast or Multicast?
Consul uses the Serf gossip protocol which relies on TCP and UDP unicast. Broadcast and Multicast are rarely available in a multi-tenant or cloud network environment. For that reason, Consul and Serf were both designed to avoid any dependence on those capabilities.
Q: Is Consul eventually or strongly consistent?
Consul has two important subsystems, the service catalog and the gossip protocol. The service catalog stores all the nodes, service instances, health check data, ACLs, and Key/Value information. It is strongly consistent, and replicated using the consensus protocol.
The gossip protocol is used to track which nodes are part of the cluster and to detect a node or agent failure. This information is eventually consistent by nature. When the servers detects a change in membership, or receive a health update, they update the service catalog appropriately.
Because of this split, the answer to the question is subtle. Almost all client APIs interact with the service catalog and are strongly consistent. Updates to the catalog may come via the gossip protocol which is eventually consistent, meaning the current state of the catalog can lag behind until the state is reconciled.
Q: Are failed or left nodes ever removed?
To prevent an accumulation of dead nodes (nodes in either failed or left states), Consul will automatically remove dead nodes out of the catalog. This process is called reaping. This is currently done on a non-configurable interval of 72 hours. Reaping is similar to leaving, causing all associated services to be deregistered.
Q: Does Consul support delta updates for watchers or blocking queries?
Consul does not currently support sending a delta or a change only response to a watcher or a blocking query. The API simply allows for an edge-trigger return with the full result. A client should keep the results of their last read and compute the delta client side.
By design, Consul offloads this to clients instead of attempting to support the delta calculation. This avoids expensive state maintenance on the servers as well as race conditions between data updates and watch registrations.