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+The cynagora protocol
+=====================
+
+Introduction
+------------
+
+### Notations:
+
+ - c->s:    from client to cynagora server
+ - s->c:    from cynagora server to client
+ - CACHEID: a 32 bits positive integer
+ - ID:      a string
+ - EXPIRE:  if missing, means: can cache forever
+            if `-`, means: don't cache
+            if TIMESPEC (see below), means: valid until given RELATIVE time
+ - SEXPIRE: Same as EXPIRE but also allows TIMESPEC prefixed with '-', meaning
+            valid until given relative time and don't cache
+
+For TIMESPEC see notes.
+
+Messages
+--------
+
+### hello
+
+synopsis:
+
+	c->s cynagora 1
+	s->c done 1 CACHEID
+
+The client present itself with the version of the protocol it expects to
+speak (today version 1 only). The server answer yes with the acknoledged
+version it will use and the CACHEID that identify the cache (see note on
+CACHEID)
+
+If hello is used, it must be the first message. If it is not used, the
+protocol implicitely switch to the default version.
+
+
+### invalidate cache
+
+synopsis:
+
+	s->c clear CACHEID
+
+The server ask the client to clear its cache and to start the cache whose
+identifier is CACHEID.
+
+This is the responsibility of the client to clear its cache. It is also a
+decision of the client to implement or not a cache. If the client implements
+a cache, it must clear that cache when it receives that message from the
+server or otherwise its decisions would be wrong.
+
+
+### test a permission
+
+synopsis:
+
+	c->s test ID CLIENT SESSION USER PERMISSION
+	s->c (ack|yes|no) ID [EXPIRE]
+
+Check whether the permission is granted (yes) or not granted (no)
+or undecidable without querying an agent (ack).
+
+This query ensure that the response is fast because agent are allowed to
+delay requests before emitting the final status. But it doesn't ensure that
+the answer is a final status. Receiving `ack` means that no final decision
+can be decided. In that case the correct resolution is either to act as if
+`no` were received or to ask for a check with not null probability that the
+reply will take time.
+
+
+### check a permission
+
+synopsis:
+
+	c->s check ID CLIENT SESSION USER PERMISSION
+	s->c (yes|no) ID [EXPIRE]
+
+Check whether the permission is granted (yes) or not granted (no) and invoke
+agent if needed.
+
+Agents are allowed to query user, remote server or any long time processing
+that may delay a lot the reply. So don't forget when using check that the
+reply might take time.
+
+
+### enter critical (admin)
+
+synopsis:
+
+	c->s enter
+	s->c done
+
+Start modifications (prior to set or drop).
+
+
+### leave critical (admin)
+
+synopsis:
+
+	c->s leave [commit|rollback]
+	s->c done|error ...
+
+Terminate modifications and commit it (commit) or cancel it (rollback).
+
+
+### erase (admin)
+
+synopsis:
+
+	c->s drop CLIENT SESSION USER PERMISSION
+	s->c done|error ...
+
+Drop the rule matching the given filter (see FILTER).
+
+
+### set (admin)
+
+synopsis:
+
+	c->s set CLIENT SESSION USER PERMISSION VALUE [SEXPIRE]
+	s->c done|error ...
+
+Create the rule as given.
+
+
+### list permissions (admin):
+
+synopsis:
+
+	c->s get CLIENT SESSION USER PERMISSION
+	s->c item CLIENT SESSION USER PERMISSION VALUE [SEXPIRE]
+	s->c ...
+	s->c done
+
+List the rules matching the given filter (see FILTER).
+
+
+### logging set/get (admin)
+
+synopsis:
+
+	c->s log [on|off]
+	s->c done (on|off)
+
+Tell to log or not the queries or query the current state.
+
+With an argument, it activates or deactivates logging. Without argument,
+it does nothing.
+
+In all cases, returns the logging state afterward.
+
+Logging is a global feature. The protocol commands that the server sends or 
+receives are printed to the journal or not.
+
+
+### register agent (agent)
+
+synopsis:
+
+	c->s agent NAME
+	s->c done|error ...
+
+Register the agent of NAME (see AGENT-NAME). The name must be valid. The
+name must not be already registered, it must be unique.
+
+
+### ask agent (agent):
+
+synopsis:
+
+	s->c ask ASKID NAME VALUE CLIENT SESSION USER PERMISSION
+	c->s reply ASKID (yes|no) [SEXPIRE]
+
+The server ask the agent of `NAME` to handle the request to the rule
+CLIENT SESSION USER PERMISSION and the agent VALUE.
+
+The agent implementation must return its result with the given associated
+ASKID. If the agent implementation has to check cynagora for replying to
+an agent request, it must use sub-check requests.
+
+
+### sub check (agent):
+
+synopsis:
+
+	c->s sub ASKID ID CLIENT SESSION USER PERMISSION
+	s->c (yes|no) ID [EXPIRE]
+
+Make a check in the context of an agent resolution. Same as `check` but
+in the context of the agent query ASKID.
+
+
+Notes
+-----
+
+### TIMESPEC
+
+The TIMESPEC describe a number of seconds in the futur RELATIVE TO NOW.
+
+It can be a simple decimal integer. I can also use letters to designate
+year (letter `y`), week (letter `w`), day (letter `d`), hour (letter `h`),
+minute (letter `m`), second (letter `s`).
+
+It can also be one of the predefined value: `*`, `forever` or `always`, all
+meaning endless.
+
+Examples:
+
+  - 15d    15 days
+  - 1h     1 hour
+  - 5m30s  5 minutes 30 seconds
+
+
+### CACHEID
+
+The cacheid identify the current cache. It changes each time the database
+changes. After a disconnection, clients can use HELLO to check whether their
+version of cache is still valid. This is implemented by the default C library.
+
+
+### FILTER
+
+The commands `drop` and `get` are taking rule's filters. A rule filter
+is a rule that accept the special character `#` as a catch all match.
+
+For examples, the rule filter `# X # #` macthes the rules that have the
+session `X` for any client, any user and any permission.
+
+
+### AGENT-NAME
+
+Agent's name are valid if it only contains the following characters:
+
+  - latin letters upper or lower case (distinguished): `a`..`z` or `A`..`Z`
+  - digits: `0`..`9`
+  - one of the characters `@`, `$`, `-`, `_`
+
+It can not be longer than 255 characters.
+
+The case count so the agent 'a' is not the agent 'A'.
+