[redis.git] / CLUSTER

CLUSTER README
==============

Redis Cluster is currently a work in progress, however there are a few things
that you can do already with it to see how it works.

The following guide show you how to setup a three nodes cluster and issue some
basic command against it.

... WORK IN PROGRESS ...

1) Show MIGRATE
2) Show CLUSTER MEET
3) Show link status detection with CLUSTER NODES
4) Show how to add slots with CLUSTER ADDSLOTS
5) Show redirection
6) Show cluster down

... WORK IN PROGRESS ...

TODO
====

*** WARNING: all the following probably has some meaning only for
*** me (antirez), most info are not updated, so please consider this file
*** as a private TODO list / brainstorming.

- disconnect FAIL clients after some pong idle time.

---------------------------------

* Majority rule: the cluster con continue when there are all the hash slots covered AND when there are the majority of masters.
* Shutdown on request rule: when a node sees many connections closed or even a timeout longer than usual on almost all the other nodes, it will usually wait for the normal timeout before to change the state, unless it receives a query from a client: in such a case it will put itself into error status.

--------------------------------

* When asked for a key that is not in a node's business it will reply:

    -ASK 1.2.3.4:6379   (in case we want the client to ask just one time)
    -MOVED <slotid> 1.2.3.4:6379  (in case the hash slot is permanently moved)

So with -ASK a client should just retry the query against this new node, a single time.

With -MOVED the client should update its hash slots table to reflect the fact that now the specified node is the one to contact for the specified hash slot.

* Nodes communicate using a binary protocol.

* Node failure detection.

    1) Every node contains information about all the other nodes:
        - If this node is believed to work ok or not
        - The hash slots for which this node is responsible
        - If the node is a master or a slave
        - If it is a slave, the slave of which node
        - if it is a master, the list of slave nodes
        - The slaves are ordered for "<ip>:<port>" string from lower to higher
          ordered lexicographically. When a master is down, the cluster will
          try to elect the first slave in the list.

    2) Every node also contains the unix time where every other node was
       reported to work properly (that is, it replied to a ping or any other
       protocol request correctly). For every node we also store the timestamp
       at which we sent the latest ping, so we can easily compute the current
       lag.

    3) From time to time a node pings a random node, selected among the nodes
       with the least recent "alive" time stamp. Three random nodes are selected
       and the one with lower alive time stamp is pinged.

    4) The ping packet contains also information about a few random nodes
       alive time stamp. So that the receiver of the ping will update the
       alive table if the received alive timestamp is more recent the
       one present in the node local table.

       In the ping packet every node "gossip" information is something like
       this:

       <ip>:<port>:<status>:<pingsent_timestamp>:<pongreceived_timestamp>

       status is OK, POSSIBLE_FAILURE, FAILURE.

    5) The node replies to ping with a pong packet, that also contains a random
       selections of nodes timestamps.

A given node thinks another node may be in a failure state once there is a
ping timeout bigger than 30 seconds (configurable).

When a possible failure is detected the node performs the following action:

    1) Is the average between all the other nodes big? For instance bigger
       than 30 seconds / 2 = 15 seconds? Probably *we* are disconnected.
       In such a case we don't trust our lag data, and reset all the
       timestamps of sent ping to zero. This way when we'll reconnect there
       is no risk that we'll claim many nodes are down, taking inappropriate
       actions.

    2) Messages from nodes marked as failed are *always* ignored by the other
       nodes. A new node needs to be "introduced" by a good online node.

    3) If we are well connected (that is, condition "1" is not true) and a
       node timeout is > 30 seconds, we mark the node as POSSIBLE_FAILURE
       (a flat in the cluster node structure). Every time we sent a ping
       to another node we inform this other nodes that we detected this
       condition, as already stated.

    4) Once a node receives a POSSIBLE_FAILURE status for a node that is
       already marked as POSSIBLE_FAILURE locally, it sends a message
       to all the other nodes of type NODE_FAILURE_DETECTED, communicating the
       ip/port of the specified node.

       All the nodes need to update the status of this node setting it into
       FAILURE.

    5) If the computer in FAILURE state is a master node, what is needed is
       to perform a Slave Election.

SLAVE ELECTION

    1) The slave election is performed by the first slave (with slaves ordered
       lexicographically). Actually it is the first functioning slave, so if
       the first slave is marked as failing the next slave will perform the
       election and so forth. Such a slave is called the "Successor".

    2) The Successor starts checking that all the nodes in the cluster already
       marked the master in FAILURE state. If at least one node does not agree
       no action is performed.

    3) If all the nodes agree that the master is failing, the Successor does
       the following:

       a) It will send a SUCCESSION message to all the other nodes, that will
          upgrade the hash slot tables accordingly. It will make sure that all
          the nodes are updated and if some node did not received the message
          it will keep trying.
       b) Once all nodes not marked as FAILURE accepted the SUCCESSION message
          it will update his own table and will start acting as a master
          accepting write queries.
       c) Every node receiving the succession message, if not already informed
          of the change will broadcast the same message to other three random
          nodes. No action is performed if the specified host was already marked
          as the master node.
       d) A node that was a slave of the original master that failed will
          switch master to the new one once the SUCCESSION message is received.

RANDOM

    1) When selecting a slave, the system will try to pick one with an IP different than the master and other slaves, if possible.

    2) The PING packet also contains information about the local configuration checksum. This is the SHA1 of the current configuration, without the bits that normally change form one node to another (like latest ping reply, failure status of nodes, and so forth). From time to time the local config SHA1 is checked against the list of the other nodes, and if there is a mismatch between our configuration and the most common one that lasts for more than N seconds, the most common configuration is asked and retrieved from another node. The event is logged.

    3) Every time a node updates its internal cluster configuration, it dumps such a config in the cluster.conf file. On startup the configuration is reloaded.
    Nodes can share the cluster configuration when needed (for instance if SHA1 does not match) using this exact same format.

CLIENTS

    - Clients may be configured to use slaves to perform reads, when read-after-write consistency is not required.
Commit	Line	Data
e6f0a7b2	1	CLUSTER README
	2	==============
	3
5215ab14	4	Redis Cluster is currently a work in progress, however there are a few things
e6f0a7b2	5	that you can do already with it to see how it works.
	6
	7	The following guide show you how to setup a three nodes cluster and issue some
	8	basic command against it.
	9
	10	... WORK IN PROGRESS ...
	11
	12	1) Show MIGRATE
	13	2) Show CLUSTER MEET
	14	3) Show link status detection with CLUSTER NODES
	15	4) Show how to add slots with CLUSTER ADDSLOTS
	16	5) Show redirection
	17	6) Show cluster down
	18
	19	... WORK IN PROGRESS ...
	20
ecc91094	21	TODO
e6f0a7b2	22	====
ecc91094	23
5215ab14	24	*** WARNING: all the following probably has some meaning only for
143d0077	25	*** me (antirez), most info are not updated, so please consider this file
	26	*** as a private TODO list / brainstorming.
	27
ecc91094	28	- disconnect FAIL clients after some pong idle time.
	29
	30	---------------------------------
	31
	32	* Majority rule: the cluster con continue when there are all the hash slots covered AND when there are the majority of masters.
	33	* Shutdown on request rule: when a node sees many connections closed or even a timeout longer than usual on almost all the other nodes, it will usually wait for the normal timeout before to change the state, unless it receives a query from a client: in such a case it will put itself into error status.
	34
	35	--------------------------------
	36
	37	* When asked for a key that is not in a node's business it will reply:
	38
	39	-ASK 1.2.3.4:6379 (in case we want the client to ask just one time)
	40	-MOVED <slotid> 1.2.3.4:6379 (in case the hash slot is permanently moved)
	41
	42	So with -ASK a client should just retry the query against this new node, a single time.
	43
	44	With -MOVED the client should update its hash slots table to reflect the fact that now the specified node is the one to contact for the specified hash slot.
	45
	46	* Nodes communicate using a binary protocol.
	47
	48	* Node failure detection.
	49
	50	1) Every node contains information about all the other nodes:
	51	- If this node is believed to work ok or not
	52	- The hash slots for which this node is responsible
	53	- If the node is a master or a slave
	54	- If it is a slave, the slave of which node
	55	- if it is a master, the list of slave nodes
	56	- The slaves are ordered for "<ip>:<port>" string from lower to higher
	57	ordered lexicographically. When a master is down, the cluster will
	58	try to elect the first slave in the list.
	59
	60	2) Every node also contains the unix time where every other node was
	61	reported to work properly (that is, it replied to a ping or any other
	62	protocol request correctly). For every node we also store the timestamp
	63	at which we sent the latest ping, so we can easily compute the current
	64	lag.
	65
	66	3) From time to time a node pings a random node, selected among the nodes
	67	with the least recent "alive" time stamp. Three random nodes are selected
	68	and the one with lower alive time stamp is pinged.
	69
	70	4) The ping packet contains also information about a few random nodes
	71	alive time stamp. So that the receiver of the ping will update the
	72	alive table if the received alive timestamp is more recent the
	73	one present in the node local table.
	74
5215ab14	75	In the ping packet every node "gossip" information is something like
ecc91094	76	this:
	77
	78	<ip>:<port>:<status>:<pingsent_timestamp>:<pongreceived_timestamp>
	79
	80	status is OK, POSSIBLE_FAILURE, FAILURE.
	81
	82	5) The node replies to ping with a pong packet, that also contains a random
	83	selections of nodes timestamps.
	84
	85	A given node thinks another node may be in a failure state once there is a
	86	ping timeout bigger than 30 seconds (configurable).
	87
	88	When a possible failure is detected the node performs the following action:
	89
	90	1) Is the average between all the other nodes big? For instance bigger
	91	than 30 seconds / 2 = 15 seconds? Probably we are disconnected.
	92	In such a case we don't trust our lag data, and reset all the
	93	timestamps of sent ping to zero. This way when we'll reconnect there
	94	is no risk that we'll claim many nodes are down, taking inappropriate
	95	actions.
	96
	97	2) Messages from nodes marked as failed are always ignored by the other
	98	nodes. A new node needs to be "introduced" by a good online node.
	99
	100	3) If we are well connected (that is, condition "1" is not true) and a
	101	node timeout is > 30 seconds, we mark the node as POSSIBLE_FAILURE
	102	(a flat in the cluster node structure). Every time we sent a ping
	103	to another node we inform this other nodes that we detected this
	104	condition, as already stated.
	105
	106	4) Once a node receives a POSSIBLE_FAILURE status for a node that is
	107	already marked as POSSIBLE_FAILURE locally, it sends a message
	108	to all the other nodes of type NODE_FAILURE_DETECTED, communicating the
	109	ip/port of the specified node.
	110
	111	All the nodes need to update the status of this node setting it into
	112	FAILURE.
	113
	114	5) If the computer in FAILURE state is a master node, what is needed is
	115	to perform a Slave Election.
	116
	117	SLAVE ELECTION
	118
	119	1) The slave election is performed by the first slave (with slaves ordered
	120	lexicographically). Actually it is the first functioning slave, so if
	121	the first slave is marked as failing the next slave will perform the
	122	election and so forth. Such a slave is called the "Successor".
	123
	124	2) The Successor starts checking that all the nodes in the cluster already
	125	marked the master in FAILURE state. If at least one node does not agree
	126	no action is performed.
	127
	128	3) If all the nodes agree that the master is failing, the Successor does
	129	the following:
	130
	131	a) It will send a SUCCESSION message to all the other nodes, that will
	132	upgrade the hash slot tables accordingly. It will make sure that all
	133	the nodes are updated and if some node did not received the message
	134	it will keep trying.
	135	b) Once all nodes not marked as FAILURE accepted the SUCCESSION message
	136	it will update his own table and will start acting as a master
	137	accepting write queries.
	138	c) Every node receiving the succession message, if not already informed
	139	of the change will broadcast the same message to other three random
140	nodes. No action is performed if the specified host was already marked
141	as the master node.
142	d) A node that was a slave of the original master that failed will
143	switch master to the new one once the SUCCESSION message is received.
144
145	RANDOM
146
147	1) When selecting a slave, the system will try to pick one with an IP different than the master and other slaves, if possible.
148
149	2) The PING packet also contains information about the local configuration checksum. This is the SHA1 of the current configuration, without the bits that normally change form one node to another (like latest ping reply, failure status of nodes, and so forth). From time to time the local config SHA1 is checked against the list of the other nodes, and if there is a mismatch between our configuration and the most common one that lasts for more than N seconds, the most common configuration is asked and retrieved from another node. The event is logged.
150
151	3) Every time a node updates its internal cluster configuration, it dumps such a config in the cluster.conf file. On startup the configuration is reloaded.
152	Nodes can share the cluster configuration when needed (for instance if SHA1 does not match) using this exact same format.
153
154	CLIENTS
155
156	- Clients may be configured to use slaves to perform reads, when read-after-write consistency is not required.