Merge pull request #165 from codex-storage/request-queue

Add request queue design

design/sales.md

@@ -1,5 +1,4 @@
-Sales module
+# Sales module
-============
 
 The sales module is responsible for selling a node's available storage in the
 [marketplace](./marketplace.md). In order to do so it needs to know how much
@@ -39,8 +38,7 @@ price to sell it for. They represent storage that is for sale, but not yet sold.
 This is information local to the node that can be altered without affecting
 global state.
 
-Adding availability
+## Adding availability
--------------------
 
 When a user adds availability, then the reservations module will check whether
 there is enough space available in the Repo. If there is enough space, then it
@@ -59,8 +57,7 @@ node is restarted.
      |                  | persist availability          |
      |                  |------------------------------>|
 
-Selling storage
+## Selling storage
----------------
 
 When a request for storage is submitted on chain, the sales module decides
 whether or not it wants to act on it. First, it tries to find an availability
@@ -105,8 +102,7 @@ updated to match the duration of the storage request.
       |------------------->| update time-to-live                |
       |                    |----------------------------------->|
 
-Ending a request
+## Ending a request
-----------------
 
 When a storage request comes to an end, then the content can be removed from the
 repo and the storage space can be made available for sale again. The same should
@@ -129,8 +125,7 @@ for the request can be re-added to the reservations module.
                            |-------------------->|              |
                            |                     |              |
 
-Persisting state
+## Persisting state
-----------------
 
 The sales module keeps state in a number of places. Most state is kept on chain,
 this includes the slots that a host is filling and the state of each slot. This
@@ -140,8 +135,79 @@ the state is kept on local disk by the Repo and the Datastore. How much space is
 reserved to be sold is persisted on disk by the Repo. The availabilities are
 persisted on disk by the Datastore.
 
-Repo
+## Request queue
-----
+
+Once a new request for storage is created on chain, all hosts will receive a
+contract event announcing the storage request and decide if they want to act on
+the request by matching their availabilities with the incoming request. Because
+there will be many requests being announced over time, each host will create a
+queue of matching requests, adding each new storage request to the queue.
+
+### Adding requests to the queue
+
+Requests will be added to the queue when request for storage events are received
+from the contracts. Additionally, when slots are freed, a contract event will
+also be received, and the request (and slot index) will be added to the queue.
+If the request of the freed slot already exists in the queue, the entry will be
+updated to include the slot index (multiple slot indices should be allowed).
+
+### Removing requests from the queue
+
+Hosts will also recieve contract events for when any contract is started,
+failed, or cancelled. In all of these cases, requests in the queue should be
+removed as they are no longer fillable by the host. Note: expired requests will
+be checked when a request is processed and its state is validated.
+
+### Sort order
+
+Requests in the queue should be sorted in the following order:
+1. Profit (descending, yet to be determined how this will be calculated)
+2. Collateral required (ascending)
+3. Time before expiry (descending)
+4. Dataset size (ascending)
+
+Note: datset size may eventually be included in the profit algorithm and may not
+need to be included in the future. Additionally, data dispersal may also impact
+the datset size to be downloaded by the host, and consequently the profitability
+of servicing a storage request, which will need to be considered in the future
+once profitability can be calculated.
+
+### Queue processing
+
+Queue processing will be started only once, when the sales module starts and
+will process requests continuously, in order, until the queue is empty. If the
+queue is empty, processing of the queue will resume once items have been added
+to the queue. If the queue is not empty, but there are no availabilities, queue
+processing will resume once availabilites have been added.
+
+When a request is processed, it first is checked to ensure there is a matching
+availabilty, as these availabilities will have changed over time. Then, a random
+slot will be chosen, and the sales process will begin. If the request being
+processed contains one or more slot indices (as in the case of repair), the
+random slot should be chosen from that sample of indices. The start of the sales
+process should ensure that the random slot chosen is indeed available (slot
+state is "free") before continuing. If it is not available, the sales process
+will exit and the host will process the top request in the queue. The start of
+the sales process should also check to ensure the host is allowed to fill the
+slot, due to the [sliding window
+mechanism](https://github.com/codex-storage/codex-research/blob/master/design/marketplace.md#dispersal).
+If the host is not allowed to fill the slot, the sales process will exit and the
+host will process the top request in the queue. Note: it may be that the top
+request in the queue is the same request that was just processed, indicating
+that the request still has slots to be filled, as the request was had not yet
+been removed due to being started/failed/cancelled. This re-processing of the
+same request allows for the possibility that the random slot index chosen had
+already been filled by another host, and gives the host a chance to fill a
+different slot index of the same request.
+
+### Implementation tips
+
+Request queue implementations should keep in mind that requests will likely need
+to be accessed randomly (by key, eg request id) and by index (for sorting), so
+implemented structures should handle these types of operations in as little time
+as possible.
+
+## Repo
 
 The Repo exposes the following functions that allow the reservations module to
 query the amount of available storage, to update the amount of reserved
@@ -153,8 +219,7 @@ space, and to store data for a guaranteed amount of time.
       function release(amount)
       function setTtl(cid, ttl)
 
-Datastore
+## Datastore
----------
 
 The Datastore is a generic key-value store that is used to persist the state of
 the Reservations module, so that it survives node restarts.