Observe

{note}The recommendations here are under development and may change before implementation.{note}

h1. Overview

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

h2. Request

{code:none}
Byte/ 0 | 1 | 2 | 3 |
/ | | | |
|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
+---------------+---------------+---------------+---------------+
0| 0x80 | 0xb1 | 0x00 | 0x00 |
+---------------+---------------+---------------+---------------+
4| 0x00 | 0x00 | 0x00 | 0x00 |
+---------------+---------------+---------------+---------------+
8| 0x00 | 0x00 | 0x00 | 0x14 |
+---------------+---------------+---------------+---------------+
12| 0xde | 0xad | 0xbe | 0xef |
+---------------+---------------+---------------+---------------+
16| 0x00 | 0x00 | 0x00 | 0x00 |
+---------------+---------------+---------------+---------------+
20| 0x00 | 0x00 | 0x00 | 0x00 |
+---------------+---------------+---------------+---------------+
24| 0x00 | 0x02 | 0x00 | 0x04 |
+---------------+---------------+---------------+---------------+
28| 0x00 | 0x05 | 0x68 ('h') | 0x65 ('e') |
+---------------+---------------+---------------+---------------+
32| 0x6c ('l') | 0x6c ('l') | 0x6f ('o') | 0x00 |
+---------------+---------------+---------------+---------------+
36| 0x05 | 0x00 | 0x05 | 0x77 ('w') |
+---------------+---------------+---------------+---------------+
40| 0x6f ('o') | 0x72 ('r') | 0x6c ('l') | 0x64 ('d') |
+---------------+---------------+---------------+---------------+
observe command
Field (offset) (value)
Magic (0) : 0x80
Opcode (1) : 0xb1
Key length (2,3) : 0x0000
Extra length (4) : 0x00
Data type (5) : 0x00
Vbucket (6,7) : 0x0000
Total body (8-11) : 0x00000014
Opaque (12-15): 0xdeadbeef
CAS (16-23): 0x0000000000000000
# Keys (24-25): 0x0002
Key #0
vbucket (26-27): 0x0004
keylen (28-29): 0x0005
(30-34): "hello"
Key #1
vbucket (35-36): 0x0005
keylen (37-38): 0x0005
(39-43): "world"
{code}

h2. Response

{code:none}
Byte/ 0 | 1 | 2 | 3 |
/ | | | |
|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
+---------------+---------------+---------------+---------------+
0| 0x81 | 0xb1 | 0x00 | 0x00 |
+---------------+---------------+---------------+---------------+
4| 0x00 | 0x00 | 0x00 | 0x00 |
+---------------+---------------+---------------+---------------+
8| 0x00 | 0x00 | 0x00 | 0x26 |
+---------------+---------------+---------------+---------------+
12| 0xde | 0xad | 0xbe | 0xef |
+---------------+---------------+---------------+---------------+
16| 0x00 | 0x00 | 0x03 | 0xe8 |
+---------------+---------------+---------------+---------------+
20| 0x00 | 0x00 | 0x00 | 0x64 |
+---------------+---------------+---------------+---------------+
24| 0x00 | 0x02 | 0x00 | 0x04 |
+---------------+---------------+---------------+---------------+
28| 0x00 | 0x05 | 0x68 ('h') | 0x65 ('e') |
+---------------+---------------+---------------+---------------+
32| 0x6c ('l') | 0x6c ('l') | 0x6f ('o') | 0x01 |
+---------------+---------------+---------------+---------------+
36| 0x00 | 0x00 | 0x00 | 0x00 |
+---------------+---------------+---------------+---------------+
40| 0x00 | 0x00 | 0x00 | 0x0a |
+---------------+---------------+---------------+---------------+
44| 0x00 | 0x05 | 0x00 | 0x05 |
+---------------+---------------+---------------+---------------+
48| 0x77 ('w') | 0x6f ('o') | 0x72 ('r') | 0x6c ('l') |
+---------------+---------------+---------------+---------------+
52| 0x64 ('d') | 0x00 | 0xde | 0xad |
+---------------+---------------+---------------+---------------+
56| 0xbe | 0xef | 0xde | 0xad |
+---------------+---------------+---------------+---------------+
60| 0xca | 0xfe |
+---------------+---------------+
observe response
Field (offset) (value)
Magic (0) : 0x81
Opcode (1) : 0xb1
Key length (2,3) : 0x0000
Extra length (4) : 0x00
Data type (5) : 0x00
Status (6,7) : 0x0000
Total body (8-11) : 0x00000026
Opaque (12-15): 0xdeadbeef
Persist Stat (16-19): 0x000003e8 (msec time)
Repl Stat (20-23): 0x00000064 (msec time)
# Keys (24-25): 0x0002
Key #0
vbucket (26-27): 0x0004
keylen (28-29): 0x0005
(30-34): "hello"
(35-35): 0x01 (persisted)
cas (36-43): 000000000000000a
Key #1
vbucket (44-45): 0x0005
keylen (46-47): 0x0005
(48-52): "world"
(53-53): 0x00 (not persisted)
cas (54-61): deadbeefdeadcafe
{code}

Note that 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.

h2. Notes

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.

h1. Client API

h2. Java

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.

{code}
OperationFuture<Boolean> setResult = cbc.set("foo", 0, "bar");

if (setResult.get()) {
Observer watchit = new Observer(setResult.getStatus());
else {
// do something because the set failed
}

// now that the observer knows what to observe...
ObserveBoolean observeSuccess = watchit.blockForReplication(1); // this will block
if (!observeSuccess) {
// handle whatever we want to do for this change
switch (observeSuccess.getStatus()) {
case OBS_TIMEDOUT:
// do something for the timeout case
break;
case OBS_MODIFIED:
// key was either modified by someone else or failover occurred
break;
}
}

{code}

h2. .NET

h2. PHP

h2. Ruby

The {{observe}} method will return false value in case of timeout and positive in case of success.

{code:none}
# observe single key and wait for 5 seconds
# until it will be replicated to at least one replica
observe("foo", :cas => 6635827497922002944, :ttl => 5, :replicas => 1)

# returns {"foo" => true, "bar" => true}
{code}

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

{code:none}
set("foo", "bar", :num_replicas => 2, :timeout => 5)
{code}

Or even better to combine these options into single option {{:observe}}. It will make more clear the fact of addition operation applied here.

{code:none}
set("foo", "bar", :observe => {:replicas => 2, :timeout => 5})
{code}

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.

{code:none}
conn.run do |c|
c.set("foo", "bar", :observe => {:replicas => 1, :persisted => 2})
c.append("baz", "bar", :observe => {:persisted => 4})
c.observe_and_wait
end
{code}

h2. C

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.

h1. Recommended Implementation
{note}These recommendations are preliminary, and have not been reviewed.{note}

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. &nbsp;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.

h3. Implementation Approach

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.

h2. Implementation Constraints

{{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.

h2. Expected use Cases

h3. Ensure Indexable
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.

h3. Ensure Durability
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).

h2. Implementation Questions
{quote}
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.
{quote}

a: ?

{quote}
q: Is a C implementation of {{OBSERVE}} needed?
{quote}