hiredis C programming for XREADGROUP

Rich

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May 21, 2020, 2:59:36 AM5/21/20

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Hi All,

I am trying to use hiredis as C redis client to program Redis stream, when I use XREADGROUP as following:

reply = redisCommand(c, "XREADGROUP GROUP diameter Tom BLOCK 1 COUNT 1 NOACK STREAMS update_db >");

it always return a NULL pointer. If I replace ">" with others like "0", reply is not NULL. it looks ">" is causing the issue. Could someone please help me to figure out how to resolve this issue?

Thank you in advance!

Rich

Itamar Haber

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May 21, 2020, 8:47:49 AM5/21/20

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Hi Rich,

Does the command return data when you run it with redis-cli?

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Rich

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May 21, 2020, 12:50:27 PM5/21/20

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Hi ltamar Haber,

Thank you for your reply. I am afraid I cannot explain my tests clearly, I listed some some test cases.

TC 1: did not add data through redis-cli before I run my program.

from redis-cli.

127.0.0.1:6379> XREADGROUP GROUP diameter Tom BLOCK 1 COUNT 1 NOACK STREAMS update_db >

(nil)

from my program:

reply = redisCommand(c, "XREADGROUP GROUP diameter Tom BLOCK 1 COUNT 1 NOACK STREAMS update_db >");

reply is NULL

TC 2: add data through redis-cli before I run my program

from redis-cli, i can get:

127.0.0.1:6379> XREADGROUP GROUP diameter Tom BLOCK 1 COUNT 1 NOACK STREAMS update_db >

1) 1) "update_db"

2) 1) 1) "1590077941505-0"

2) 1) "pri_key"

2) "1006"

(add data again because data was read by redis-cli)

from my program:

reply = redisCommand(c, "XREADGROUP GROUP diameter Tom BLOCK 1 COUNT 1 NOACK STREAMS update_db >");

size_t m = 0;

if (reply != NULL) {

for (m = 0; m < reply->elements; m++)

printf("XREADGROUP %s\n", reply->element[m]->str);

}

reply is not NULL but I did not get any data, output is printed out but it is "null". it looks reply->element[m]->str contains null.

TC 3, change BLOCK to be 0, expect program to block until new data comes.

from redis-cli,

the command: "XREADGROUP GROUP diameter Tom BLOCK 0 COUNT 1 NOACK STREAMS update_db >" would block, when the new data comes, it will read data and return.

However from program, I used:

reply = redisCommand(c, "XREADGROUP GROUP diameter Tom BLOCK 0 COUNT 1 NOACK STREAMS update_db >");

I am hoping it would block and wait for the new data (before running my program, I clean up the data by using flushall from redis-cli), but it actually did not block as expected, it return and reply is NULL.

Thank you,

Rich

On Thursday, May 21, 2020 at 8:47:49 AM UTC-4, Itamar Haber wrote:

Hi Rich,

Does the command return data when you run it with redis-cli?

On Thu, May 21, 2020 at 9:59 AM Rich <richar...@gmail.com> wrote:

Hi All,
I am trying to use hiredis as C redis client to program Redis stream, when I use XREADGROUP as following:

reply = redisCommand(c, "XREADGROUP GROUP diameter Tom BLOCK 1 COUNT 1 NOACK STREAMS update_db >");

it always return a NULL pointer. If I replace ">" with others like "0", reply is not NULL. it looks ">" is causing the issue. Could someone please help me to figure out how to resolve this issue?

Thank you in advance!

Rich

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Itamar Haber

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May 21, 2020, 12:59:12 PM5/21/20

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> TC 3, change BLOCK to be 0 ... However from program, I used...

Are you absolutely, positively and certainly sure that you ran the re-compile code in your test? Returning null indicates block's timeout, and my usual mistake is fixing the code but running an old artifact.

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Rich

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May 21, 2020, 1:07:45 PM5/21/20

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yes, I just delete the binary and re-compile it. same result.

reply = redisCommand(c, "XREADGROUP GROUP diameter Tom BLOCK 0 COUNT 1 NOACK STREAMS update_db >"); would not block and return with reply = NULL.

Thank you,

Rich

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Itamar Haber

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May 21, 2020, 1:15:46 PM5/21/20

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Weird - we must be both missing something obvious :)

I suggest you open an issue at https://github.com/redis/hiredis for help, with a minimal reproducible example.

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Rich

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May 21, 2020, 1:18:01 PM5/21/20

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yes, here are the steps I verified.

1. make sure there is no data in redis, and then run "XREADGROUP GROUP diameter Tom BLOCK 0 COUNT 1 NOACK STREAMS update_db >"

127.0.0.1:6379> XREADGROUP GROUP diameter Tom BLOCK 0 COUNT 1 NOACK STREAMS update_db > ----> just wait here

2. from redis-cli to add data

127.0.0.1:6379> XADD update_db * pri_key 1012

"1590081349128-0"

3. check the result and that command return.

127.0.0.1:6379> XREADGROUP GROUP diameter Tom BLOCK 0 COUNT 1 NOACK STREAMS update_db >

1) 1) "update_db"

2) 1) 1) "1590081349128-0"

2) 1) "pri_key"

2) "1012"

(185.48s)

Thank you,

Rich

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May 21, 2020, 1:24:54 PM5/21/20

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sure, I will open a issue.

Thank you,

Rich

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Jonathan Mendoza

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May 21, 2020, 2:07:38 PM5/21/20

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Did you create the consumer group *after* the first message was sent? If so, then the client wont consider that a *new* message that the consumer should read.

In other words:

XADD msg
XGROUP CEATE

XREADGROUP

leads to Null

However:

XGROUP CEATE

XADD msg

XREADGROUP

should pick the message up.

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Rich

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May 21, 2020, 2:39:12 PM5/21/20

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Hi Jonathan,

Thank you for your information.

I tried both way and the message still can not be picked up. Here is my code pieces.

printf("create a consumer group\n");

reply = redisCommand(c, "XGROUP CREATE update_db diameter $ MKSTREAM");

printf("XGROUP create %s\n", reply->str);

freeReplyObject(reply);

reply = redisCommand(c, "XADD update_db * %b %b", "pri_key",(size_t)7, "customer:1005", (size_t)13);

if (reply != NULL)

{

printf("XADD id is %s\n", reply->str);

}

freeReplyObject(reply);

/*create consumer*/

reply = redisCommand(c, "XREADGROUP GROUP diameter Tom BLOCK 0 COUNT 1 NOACK STREAMS update_db >");

size_t m = 0;

if (reply != NULL) {

for (m = 0; m < reply->elements; m++)

printf("XREADGROUP %s\n", reply->element[m]->str);

}

freeReplyObject(reply);

when I ran it, the result was:

create a consumer group

XGROUP create OK

XADD id is 1590085928884-0

XREADGROUP (null)

both XGROUP and XADD were successful but XREADGROUP failed to pick up message.

Thank you,

Rich

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