Multi-Database Support

[TalZaccai]

This PR enables multiple logical databases in a single Garnet server instance.

This PR includes:

• Adding MaxDatabases configuration to garnet.conf + compatibility with redis.conf
• Multi-DB support in StoreWrapper, including checkpointing & AOF
• Current-DB context switching support in RespServerSession
• Update GarnetInfoMetrics to reflect multiple databases data
• Support for the following commands:
  • SELECT - switch current DB context
  • SWAPDB - swap two DB instances
  • FLUSHDB - flush all keys from current DB in context
  • FLUSHALL - flush all keys from all active databases
• Add support for specifying a DB ID in the following commands:
  • SAVE - synchronous checkpointing database by ID
  • BGSAVE - asynchronous checkpointing database by ID
  • LASTSAVE - number of seconds since last checkpoint of database by ID
  • COMMITAOF - synchronous database AOF commit by ID
• Add DB ID to client info
• Testing for supported commands
• Benchmarking for supported commands
• Add website docs for new commands

[prvyk]

Some tiny nitpicking if it's not too early:

A) Some more bounds checking can be done on the lower bound.
e.g. 'LASTSAVE -1' or starting with --max-databases=0 (both trigger an assertion).

B) SWAPDB doesn't update CLIENT LIST db field if the current connection's database is swapped. Also doesn't update the current database when called from lua inside the script context.

C) Redis also adds a line for each db under 'Keyspace' in INFO command.

[TalZaccai]

Some tiny nitpicking if it's not too early:

A) Some more bounds checking can be done on the lower bound. e.g. 'LASTSAVE -1' or starting with --max-databases=0 (both trigger an assertion).

B) SWAPDB doesn't update CLIENT LIST db field if the current connection's database is swapped. Also doesn't update the current database when called from lua inside the script context.

C) Redis also adds a line for each db under 'Keyspace' in INFO command.

Thanks @prvyk! Super helpful!
A - fixed.
B - not sure what you mean here, say the active DB is 1 and we do a SWAPDB 0 1, the active db is still 1 (and the CLIENT LIST sould show so).
C - we currently don't support keyspace, even when we had just one logical database, I'm assuming it's because it's a fairly slow operation (since it requires scanning through all keys in each store), so I left it as-is.

[prvyk]

Thanks @prvyk! Super helpful!

A - fixed.

In some cases the check is "db < -1" (and not db < 0). What is -1 supposed to mean when it's 'legal'?

B - not sure what you mean here

I think I got confused with CLIENT LIST. The lua thing is this:

SET a 0
SELECT 2
SET a 2
SELECT 0
EVAL "return { redis.call('SWAPDB', 0, 2); redis.call('GET', 'a') }" 0

Redis ->

1. OK
2. "2"

Garnet ->

1. "OK"
2. "0"

(Afterwards, 'GET a' returns "2" with both)

Redis seems to switch during the script context.

[TalZaccai]

Thanks @prvyk! Super helpful!

A - fixed.

In some cases the check is "db < -1" (and not db < 0). What is -1 supposed to mean when it's 'legal'?

-1 is the same as not specifying an ID (i.e. "save all etc."), I suppose I can just disable it from the input as to not cause confusion.

B - not sure what you mean here

I think I got confused with CLIENT LIST. The lua thing is this:

SET a 0
SELECT 2
SET a 2
SELECT 0
EVAL "return { redis.call('SWAPDB', 0, 2); redis.call('GET', 'a') }" 0

Redis ->

1. OK
2. "2"

Garnet ->

1. "OK"
2. "0"

(Afterwards, 'GET a' returns "2" with both)

Redis seems to switch during the script context.

Good to know, I haven't tested SWAPDB from a LUA context, I'll add that to the tests as well!

[TalZaccai]

@prvyk fixed the Lua swap issue, let me know if you see anything else that needs attention :) Thanks again!