|The recommendations here are under development and may change before implementation.|
The intent of the OBSERVE operation, as implemented by Couchbase client libraries, is to give developers the ability to observe the status of a key with respect to a specific change. For example, a developer may want to somehow mutate a document behind the key identified by the string "foo". Given that a Couchbase cluster is distributed and concurrent and acknowledges the request before the given document, there are situations where the application developer may want to observe whether the specific mutation they made is either:
- safely replicated to one or more nodes within the cluster
- persisted to the master, or persisted to one or more nodes within the cluster
- has been taken into consideration in any indexes
In the response, the keystate field is intended to be a bitmask for various states. For example:
0x80 = not found, not persisted
0x00 = found, not persisted
0x01 = found, persisted
0x81 << an impossible condition
This is primarily because replication can potentially be significantly behind the initial create and we do not want to receive a NOT_FOUND error for the entire set of items being observed in that state. The intent of this being a bitmask is that future extension could allow for this field to indicate if the item is considered in any indexes, etc. Error masks start from the top of the field.
Also, in the response above, the field normally used for CAS has been separated into two fields which are to provide statistics on the average amount of time for certain events to propagate from a given server. The first is the amount of time, in milliseconds, it's taking on average to persist items. The second is the amount of time, in milliseconds, it's taking to replicate items. If not implemented by a given server version, each of these fields would be padded with 0. These fields allow a client library to optionally be a bit more intelligent, and thus efficient, about its polling intervals. The examples above show 1000 milliseconds to persistence and 100 milliseconds to replication. Note that this is a server's best guess on how long things take on average for any given operation, it is not related to the specific key being observed.
The client build a request packet once and put there all keys it interested in. After that it sends to all servers owning vbuckets (master and replicas). The servers should send back response with only keys he knows about.
The Java client library, and it's dependencies, will require changes to return the CAS value through the OperationStatus object.
The basic idea is that given the OperationStatus object (or optionally, multiple OperationStatus objects), an Observer can report on whether or not the operation has been replicated, persisted, or processed for indexing.
The existing .NET Client Library mutate methods will be overloaded to include two new options. The first option will be for specifying the number of replicas to which a key is written before the operation is considered a success. The second option will be to specify how whether a write must be persisted to be considered successful.
In the examples above, the ReplicateTo and PersistTo are enums with values from 1 to 4. PersistLevel is an enum with values of Memory and Disk.
The original mutate operations defined in the MemcachedClient returned only Boolean values. Because of the need to report a 'modified' result, only the ExecuteXXX mutate methods will be modified.
The observe method will return false value in case of timeout and positive in case of success.
As alternative API all mutators (i.e. set/add/replace/append/prepend) should have two new options:
- :num_replicas — how many replicas desired to consider mutator successful. as far as the client knows how many replicas configured, it could also raise ArgumentError exception if this option negative or excessive.
- :timeout — the timeout for replication. In this case it would be better to raise exception instead of returning falsy value
Or even better to combine these options into single option :observe. It will make more clear the fact of addition operation applied here.
Because the code above written in synchronous fashion, it is ok to wait before method will return the value. But in asynchronous mode we can return immediately.
For C client (libcouchbase) it will be better to implement only low-level operation because of its asynchronous nature. This means the there should be libcouchbase_observe() function accepting the key and returning its status on all replicas into the corresponding callback.
|These recommendations are preliminary, and have not been reviewed.|
Given that the item being mutated may be changed at any time by another actor in a deployment, the key idea here is for the client to use the CAS value returned from the mutation operation. Since the client library knows at all times (though, asynchronously) which node is responsible for the vbucket for a given key, and knows which nodes are slaves for that key, the client library can use the OBSERVE command as a method of determining what has happened with a key on this given node.
If, for example, application code wanted to check for "foo" with CAS value 12345, it would use OBSERVE command against whichever nodes it needs to in order to report the status. This would be done in a loop, with some reasonable backoff (possibly guided by recommended polling times from server stats) until a reasonable or user specified timeout.
Looping steps would consist of...
First the client would check to ensure that "foo" 12345 is still the current value on the master for this vbucket via OBSERVE. If it is not the current value, it simply returns saying the item is modified.
If it is still the current value, the next step for the client depends whether the application code is simply checking for persistence or index processing. This can be evaluated from the OBSERVE response. If the application code is checking for replication or whether or not the modification has been persisted on multiple nodes, it can then proceed to check any slaves, as identified by the cluster configuration, for status of persistence or replication of "foo" with CAS 12345. If successful, it returns that response to the application code. If it is not successful, then it waits an interval and loops again, blocking the application code until status is determined or a timeout value has been reached.
Thus the possible set of return values would be either OBS_SUCCESS, OBS_MODIFIED, or OBS_TIMEDOUT.
Note that the status OBS_MODIFIED does not indicate monotonic forward mutation. For example, in one scenario a failover may have occurred and the item key "foo" being observed may have been reverted to a previous state. This state may even be some value prior to the initial fetch before the application code mutated the value of the document.
OBSERVE is a binary protocol only operation. It could be implemented in ASCII, but that would currently be complicated by the fact that mutation operations do not return the new CAS value in ASCII protocol. Since Couchbase Server uses binary protocol exclusively, we do not implement that currently.
The current implementation of Couchbase Server does not consider items in stale=false view queries until items are persisted. The combination of an application making changes, ensuring they're persisted and then issuing a stale=false query will give the app an ability to consistently access the view with respect to any changes that application made.
Further information on this use case is to be documented elsewhere.
Through client library specific APIs, applications may wish to verify that the change recently made was durable. For example, in some cases a user application may wish to verify a data item has been made on the master and at least one replica. Another example may be that a user application may be extremely paranoid that a particular data update not be lost. In that case, it would want to ensure that item has been replicated to at least 3 servers and persisted to at least 4 servers (the maximum durability supported by Couchbase Server).
q: Is it better to return the values OBS_SUCCESS, OBS_MODIFIED, and OBS_TIMEDOUT instead of true/false and treating the timeout as an exceptional condition? Since OBS_MODIFIED and OBS_TIMEDOUT may effectively require the same error handling, it may be easier to switch on these or return some extended boolean with status.
q: Is a C implementation of OBSERVE needed?