The document provides instructions for building a Redis module that implements a ZPOP command to pop and remove the first element of a sorted set. It includes code for the ZPOP command implementation, registering the module and command, and compiling the module into a shared object library that can be loaded by Redis. The document also suggests some questions to consider before taking on developing a new module, such as whether the capability already exists or an open source module could be reused.

1. Day 1
Itamar Haber @itamarhaber #Redis #RedisConf

2. hello I am

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6. No
Gave serious
thought about
what you need
to solve?
Is there a core
Redis capability
that does it?
Do that!
Can you do it in
the app?
Is Lua good
enough?
Is there an
existing GA
module that
already does it?
Yes
No
No No
Honestly
factored cost of
taking on a new
software
project?
Yes Yes Yes
No
Is it a valid
feature request
to the core?
Yes
Yes
No
mebi
Roll
out
your
own
Try that first.
Hackers love
helping each
other 3:-)

7. Found a really
good name for
it, something
catchy and
cool?
Go back one
diagram. Start
at the beginning
mebi

8. ctx
argv argc
#include "redismodule.h"
int MyCommand(RedisModuleCtx *ctx,
RedisModuleString **argv, int argc) {
// My code here
// ...
return REDISMODULE_OK;
}

9. int ZPop(RedisModuleCtx *ctx, RedisModuleString **argv, int argc) {
if (argc != 2) {
RedisModule_WrongArity(ctx);
return REDISMODULE_OK;
}
RedisModule_AutoMemory(ctx);
RedisModuleCallReply *rep = RedisModule_Call(
ctx, "ZRANGE", "!sllc", argv[1], 0, 0, "WITHSCORES");
if (RedisModule_CallReplyType(rep) == REDISMODULE_REPLY_ERROR) {
RedisModule_ReplyWithCallReply(ctx, rep);
return REDISMODULE_OK;
}
RedisModuleString *ele =
RedisModule_CreateStringFromCallReply(
RedisModule_CallReplyArrayElement(arr, 0));
RedisModule_Call(ctx, "ZREM", "ss", key, ele);
RedisModule_ReplyWithCallReply(ctx, rep);
return REDISMODULE_OK;
}

10. // Registering the module and its commands
int RedisModule_OnLoad(RedisModuleCtx *ctx,
RedisModuleString **argv, int argc) {
if (RedisModule_Init(ctx, "zpop", 1,
REDISMODULE_APIVER_1) == REDISMODULE_ERR) {
return REDISMODULE_ERR;
}
if (RedisModule_CreateCommand(ctx,
"z.pop", ZPop, "Write", 1, 1, 1) ==
REDISMODULE_ERR) {
return REDISMODULE_ERR;
}
return REDISMODULE_OK;
}

11. # Compile it on Linux:
$ gcc -fPIC -std=gnu99 -c -o zpop.o zpop.c
$ ld -o zpop.so zpop.o -shared -Bsymbolic -lc
# Compile it on OSX:
$ gcc -dynamic -fno-common -std=gnu99 -c -o zpop.o zpop.c
$ ld -o zpop.so zpop.o -bundle -undefined dynamic_lookup
-lc
# Run it:
$ redis-server --loadmodule ./zpop.so
# Use it:
$ redis-cli
redis> ZADD z 0 a 1 b 3 c
(integer) 3
redis> Z.POP z
"a"

12. RedisModule_CallReply

13. RedisModule_WrongArity()
RedisModule_Call()

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