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Subscribing
Functions » JetStream

## Functions

NATS_EXTERN natsStatus | jsSubOptions_Init (jsSubOptions *opts)
| Initializes a subscribe options structure.
NATS_EXTERN natsStatus | js_Subscribe (natsSubscription **sub, jsCtx *js, const char *subject, natsMsgHandler cb, void *cbClosure, jsOptions *opts, jsSubOptions *subOpts, jsErrCode *errCode)
| Create an asynchronous subscription.
NATS_EXTERN natsStatus | js_SubscribeSync (natsSubscription **sub, jsCtx *js, const char *subject, jsOptions *opts, jsSubOptions *subOpts, jsErrCode *errCode)
| Create a synchronous subscription.
NATS_EXTERN natsStatus | js_PullSubscribe (natsSubscription **sub, jsCtx *js, const char *subject, const char *durable, jsOptions *opts, jsSubOptions *subOpts, jsErrCode *errCode)
| Create a pull subscriber.
NATS_EXTERN natsStatus | natsSubscription_Fetch (natsMsgList *list, natsSubscription *sub, int batch, int64_t timeout, jsErrCode *errCode)
| Fetches messages for a pull subscription.
NATS_EXTERN natsStatus | jsFetchRequest_Init (jsFetchRequest *request)
| Initializes a fetch request options structure.
NATS_EXTERN natsStatus | natsSubscription_FetchRequest (natsMsgList *list, natsSubscription *sub, jsFetchRequest *request)
| Fetches messages for a pull subscription with a complete request configuration.
NATS_EXTERN natsStatus | natsSubscription_GetConsumerInfo (jsConsumerInfo **ci, natsSubscription *sub, jsOptions *opts, jsErrCode *errCode)
| Returns the jsConsumerInfo associated with this subscription.
NATS_EXTERN natsStatus | natsSubscription_GetSequenceMismatch (jsConsumerSequenceMismatch *csm, natsSubscription *sub)
| Returns the consumer sequence mismatch information.

## Detailed Description

Subscribing functions

## Function Documentation

## jsSubOptions_Init()

NATS_EXTERN natsStatus jsSubOptions_Init | ( | jsSubOptions * | opts | ) |

Use this before setting specific subscribe options and passing this configuration to the JetStream subscribe APIs.

Parameters

  opts | the pointer to the jsSubOptions to initialize.

## js_Subscribe()

NATS_EXTERN natsStatus js_Subscribe | ( | natsSubscription ** | sub,
|  | jsCtx * | js,
|  | const char * | subject,
|  | natsMsgHandler | cb,
|  | void * | cbClosure,
|  | jsOptions * | opts,
|  | jsSubOptions * | subOpts,
|  | jsErrCode * | errCode
| ) |  |

Typically the user or administrator will have created a JetStream consumer and this call will reference the stream/consumer to bind to with the use of jsSubOptions's `Stream` and `Consumer`.

Without the stream information, the library will use the provided `subject` to try figure out which stream this subscription is for.

If a `Durable` is specified (with jsSubOptions' `Config.Durable`), the subscription will be durable. However, note the behavior described below regarding JetStream consumers created by this call.

If no `Durable` is specified, the subscription will be ephemeral and removed by the server either after calling natsSubscription_Unsubscribe or after the subscription is destroyed and the `InactivityThreshold` has elapsed.

Note
  If a JetStream consumer does not exist and this call creates it, it will be removed in the server once the user calls natsSubscription_Unsubscribe or natsSubscription_Drain, even if this is a `Durable` subscription. If the subscription should be maintained, it should be explicitly created using js_AddConsumer and then bound to with the use of jsSubOptions' `Stream` and `Consumer`.

Warning
  Prior to release v3.4.0, calling natsSubscription_Destroy would delete the JetStream consumer if it was created by this call. The original intent was that it would be deleted only with explicit calls to unsubscribe or drain. Therefore, starting v3.4.0, if the user calls only natsSubscription_Destroy (to free memory), the JetStream consumer will no longer be deleted. The user would have to explicitly call natsSubscription_Unsubscribe or js_DeleteConsumer.

Parameters

  sub | the location where to store the pointer to the newly created natsSubscription object.
  js | the pointer to the jsCtx object.
  subject | the subject this subscription is created for.
  cb | the natsMsgHandler callback.
  cbClosure | a pointer to an user defined object (can be `NULL`). See the natsMsgHandler prototype.
  opts | the pointer to the jsOptions object, possibly `NULL`.
  subOpts | the subscribe options, possibly `NULL`.
  errCode | the location where to store the JetStream specific error code, or `NULL` if not needed.

## js_SubscribeSync()

NATS_EXTERN natsStatus js_SubscribeSync | ( | natsSubscription ** | sub,
|  | jsCtx * | js,
|  | const char * | subject,
|  | jsOptions * | opts,
|  | jsSubOptions * | subOpts,
|  | jsErrCode * | errCode
| ) |  |

See important notes in js_Subscribe.

Parameters

  sub | the location where to store the pointer to the newly created natsSubscription object.
  js | the pointer to the jsCtx object.
  subject | the subject this subscription is created for. the natsMsgHandler prototype.
  opts | the pointer to the jsOptions object, possibly `NULL`.
  subOpts | the subscribe options, possibly `NULL`.
  errCode | the location where to store the JetStream specific error code, or `NULL` if not needed.

## js_PullSubscribe()

NATS_EXTERN natsStatus js_PullSubscribe | ( | natsSubscription ** | sub,
|  | jsCtx * | js,
|  | const char * | subject,
|  | const char * | durable,
|  | jsOptions * | opts,
|  | jsSubOptions * | subOpts,
|  | jsErrCode * | errCode
| ) |  |

A pull based consumer is a type of consumer that does not have a delivery subject, that is the library has to request for the messages to be delivered as needed from the server.

Note
  If no durable name is provided, the pull subscription will create an ephemeral JetStream consumer. The requirement for a durable name is lifted in NATS C client v3.4.0+ and NATS Server v2.7.0+.
  If a durable name is specified, it cannot contain the character ".".

See important notes in js_Subscribe.

Parameters

  sub | the location where to store the pointer to the newly created natsSubscription object.
  js | the pointer to the jsCtx object.
  subject | the subject this subscription is created for.
  durable | the optional durable name.
  opts | the pointer to the jsOptions object, possibly `NULL`.
  subOpts | the subscribe options, possibly `NULL`.
  errCode | the location where to store the JetStream specific error code, or `NULL` if not needed.

## natsSubscription_Fetch()

NATS_EXTERN natsStatus natsSubscription_Fetch | ( | natsMsgList * | list,
|  | natsSubscription * | sub,
|  | int | batch,
|  | int64_t | timeout,
|  | jsErrCode * | errCode
| ) |  |

Fetches up to `batch` messages from the server, waiting up to `timeout` milliseconds. No more thant `batch` messages will be returned, however, it can be less.

For `batch` greater than `1`, this call will not necessarily wait up `timeout` milliseconds if some messages were collected and the library receives a notification that no more messages are available at this time.
It means that calling `natsSubscription_Fetch(&list, sub, 10, 5000)` may return after less than 5 seconds with only 3 messages.

Parameters

  list | the location to a natsMsgList that will be filled by the result of this call.
  sub | the pointer to the natsSubscription object.
  batch | the batch size, that is, the maximum number of messages to return.
  timeout | the timeout (required) expressed in number of milliseconds.
  errCode | the location where to store the JetStream specific error code, or `NULL` if not needed.

## jsFetchRequest_Init()

NATS_EXTERN natsStatus jsFetchRequest_Init | ( | jsFetchRequest * | request | ) |

Use this before setting specific fetch options and passing it to natsSubscription_FetchRequest.

Parameters

  request | the pointer to the jsFetchRequest object.

## natsSubscription_FetchRequest()

NATS_EXTERN natsStatus natsSubscription_FetchRequest | ( | natsMsgList * | list,
|  | natsSubscription * | sub,
|  | jsFetchRequest * | request
| ) |  |

Similar to natsSubscription_Fetch but a full jsFetchRequest configuration is provided for maximum control.

Initialize the jsFetchRequest structure using jsFetchRequest_Init and then set the parameters desired, then invoke this function.

Parameters

  list | the location to a natsMsgList that will be filled by the result of this call.
  sub | the pointer to the natsSubscription object.
  request | the pointer to a jsFetchRequest configuration.

## natsSubscription_GetConsumerInfo()

NATS_EXTERN natsStatus natsSubscription_GetConsumerInfo | ( | jsConsumerInfo ** | ci,
|  | natsSubscription * | sub,
|  | jsOptions * | opts,
|  | jsErrCode * | errCode
| ) |  |

Returns the jsConsumerInfo associated with this subscription.

Parameters

  ci | the location where to store the pointer to the new jsConsumerInfo object.
  sub | the pointer to the natsSubscription object.
  opts | the pointer to the jsOptions object, possibly `NULL`.
  errCode | the location where to store the JetStream specific error code, or `NULL` if not needed.

## natsSubscription_GetSequenceMismatch()

NATS_EXTERN natsStatus natsSubscription_GetSequenceMismatch | ( | jsConsumerSequenceMismatch * | csm,
|  | natsSubscription * | sub
| ) |  |

If `Heartbeat` is configured in jsConsumerConfig object (or configured in an existing JetStream consumer), the server sends heartbeats to the client at the given interval.

Those heartbeats contains information that allow the application to detect a mismatch between the server and client's view of the state of the consumer.

If the library detects a sequence mismatch, the behavior is different depending on the type of subscription:

- For asynchronous subscriptions: the error NATS_MISMATCH is published to the error handler (see natsOptions_SetErrorHandler).
- For synchronous subscriptions: the next call to natsSubscription_NextMsg() will return the error NATS_MISMATCH (but will succeed afterwards).

In both cases, the user should check what the mismatch is using this function and possibly recreate the consumer based on the provided information.

Note
  For a valid JetStream subscription, this function will return NATS_NOT_FOUND if no consumer sequence mismatch has been detected.

See also
  jsConsumerSequenceMismatch

Parameters

  csm | the pointer location where to copy the consumer sequence mismatch information.
  sub | the pointer to the natsSubscription object.