Module membership

This module is a membership library for Tarantool based on a gossip protocol.

This library builds a mesh from multiple Tarantool instances. The mesh monitors itself, helps members discover everyone else in the group and get notified about their status changes with low latency. It is built upon the ideas from Consul or, more precisely, the SWIM algorithm.

The membership module works over UDP protocol and can operate even before the box.cfg initialization.

A member is represented by the table with the following fields:

• uri (string) is a Uniform Resource Identifier.
• status (string) is a string that takes one of the values below.

  • alive: a member that replies to ping-messages is alive and well.

  • suspect: if any member in the group cannot get a reply from any other member, the first member asks three other alive members to send a ping-message to the member in question. If there is no response, the latter becomes a suspect.

  • dead: a suspect becomes dead after a timeout.

  • left: a member gets the left status after executing the leave() function.

• incarnation (number) is a value incremented every time the instance becomes a suspect, dead, or updates its payload.
• payload (table) is auxiliary data that can be used by various modules.
• timestamp (number) is a value of fiber.time64() which:
  • corresponds to the last update of status or incarnation;
  • is always local;
  • does not depend on other members' clock setting.

Below is an example of the table:

 
tarantool> membership.myself()---uri: localhost:33001status: aliveincarnation: 1payload:    uuid: 2d00c500-2570-4019-bfcc-ab25e5096b73timestamp: 1522427330993752...

Below is a list of membership's common, encryption, subscription functions, and options.

membership

Common functions:

Initialize the membership module. This binds a UDP socket to 0.0.0.0:<port>, sets the advertise_uri parameter to <advertise_host>:<port>, and incarnation to 1.

The init() function can be called several times, the old socket will be closed and a new one opened.

If the advertise_uri changes during the next init(), the old URI is considered DEAD. In order to leave the group gracefully, use the leave() function.

Parameters:

• advertise_host (string) — a hostname or IP address to advertise to other members

• port (number) — a UDP port to bind

Returns

true

Return type

boolean

raises : socket bind error

Returns

the member data structure of the current instance.

Return type

table

Parameters:

• uri (string) — the given member's advertise_uri

Returns

the member data structure of the instance with the given URI.

Return type

table

Obtain all members known to the current instance.

Editing this table has no effect.

Returns

a table with URIs as keys and corresponding member data structures as values.

Return type

table

A shorthand for pairs(membership.members()).

Returns

Lua iterator

It can be used in the following way:

 
for uri, member in membership.pairs()  -- do somethingend

Add a member with the given URI to the group and propagate this event to other members. Adding a member to a single instance is enough as everybody else in the group will receive the update with time. It does not matter who adds whom.

Parameters:

• uri (string) — the advertise_uri of the member to add

Returns

true or nil in case of an error

Return type

boolean

raises : parse error if the URI cannot be parsed

Send a message to a member to make sure it is in the group. If the member is alive but not in the group, it is added. If it already is in the group, nothing happens.

Parameters:

• uri (string) — the advertise_uri of the member to ping

Returns

true if the member responds within 0.2 seconds, otherwise no response

Return type

boolean

raises : ping was not sent if the hostname could not be resolved

Discover members in local network by sending a UDP broadcast message to all networks discovered by a getifaddrs() C call.

Returns

true if broadcast was sent, false if getaddrinfo() fails.

Return type

boolean

Update myself().payload and disseminate it along with the member status.

Increments incarnation.

Parameters:

• key (string) — a key to set in payload table

• value — auxiliary data

Returns

true

Return type

boolean

Gracefully leave the membership group. The node will be marked with the left status and no other members will ever try to reconnect it.

Returns

true

Return type

boolean

Returns

true if encryption is enabled, false otherwise.

Return type

boolean

Encryption functions:

Set the key used for low-level message encryption. The key is either trimmed or padded automatically to be exactly 32 bytes. If the key value is nil, the encryption is disabled.

The encryption is handled by the crypto.cipher.aes256.cbc Tarantool module.

For proper communication, all members must be configured to use the same encryption key. Otherwise, members report either dead or non-decryptable in their status.

Parameters:

• key (string) — encryption key

Returns

nil.

Retrieve the encryption key that is currently in use.

Returns

encryption key or nil if the encryption is disabled.

Return type

string

Subscription functions:

Subscribe for updates in the members table.

Returns

a fiber.cond object broadcasted whenever the members table changes.

Return type

object

Remove subscription on cond obtained by the subscribe() function.

The cond's validity is not checked.

Parameters:

• cond — the fiber.cond object obtained from subscribe()

Returns

nil.

Below is a list of membership options. They can be set as follows:

 
options = require('membership.options')options.<option> = <value>

Period of sending direct pings. Denoted as T' in the SWIM protocol.

Time to wait for ACK message after a ping. If a member is late to reply, the indirect ping algorithm is invoked.

Period to perform the anti-entropy synchronization algorithm of the SWIM protocol.

Timeout to mark suspect members as dead.

Number of members to try pinging a suspect indirectly. Denoted as k in the SWIM protocol.