{"Package":"liteq","Title":"Lightweight Portable Message Queue Using 'SQLite'","Version":"1.0.1","Author":"Gbor Csrdi","Maintainer":"Gbor Csrdi ","Description":"Temporary and permanent message queues for R. Built on top of\n'SQLite' databases. 'SQLite' provides locking, and makes it possible\nto detect crashed consumers. Crashed jobs can be automatically marked\nas \"failed\", or put in the queue again, potentially a limited number of times.","License":"MIT + file LICENSE","LazyData":"true","URL":"https://github.com/r-lib/liteq#readme","BugReports":"https://github.com/r-lib/liteq/issues","RoxygenNote":"6.0.1","Imports":"assertthat, DBI, rappdirs, RSQLite","Suggests":"covr, testthat, withr","Encoding":"UTF-8","NeedsCompilation":"no","Packaged":"2017-10-20 17:39:48 UTC; gaborcsardi","Repository":"CRAN","Date/Publication":"2017-10-20 19:47:15 UTC","repoType":"cran","tarballUrl":"ftp://cran.r-project.org/pub/R/src/contrib/liteq_1.0.1.tar.gz","readme":"\n\n\n# liteq\n\n> Lightweight Portable Message Queue Using SQLite\n\n[![Linux Build Status](https://travis-ci.org/r-lib/liteq.svg?branch=master)](https://travis-ci.org/r-lib/liteq)\n[![Windows Build status](https://ci.appveyor.com/api/projects/status/github/r-lib/liteq?svg=true)](https://ci.appveyor.com/project/gaborcsardi/liteq)\n[![](http://www.r-pkg.org/badges/version/liteq)](http://www.r-pkg.org/pkg/liteq)\n[![CRAN RStudio mirror downloads](http://cranlogs.r-pkg.org/badges/liteq)](http://www.r-pkg.org/pkg/liteq)\n[![Coverage Status](https://img.shields.io/codecov/c/github/r-lib/liteq/master.svg)](https://codecov.io/github/r-lib/liteq?branch=master)\n\nTemporary and permanent message queues for R. Built on top of SQLite\ndatabases. 'SQLite' provides locking, and makes it possible to detect\ncrashed consumers. Crashed jobs can be automatically marked as \"failed\",\nor put back in the queue again, potentially a limited number of times.\n\n## Installation\n\n```r\nsource(\"https://install-github.me/r-lib/liteq\")\n```\n\n## Introduction\n\n`liteq` implements a serverless message queue system in R.\nIt can handle multiple databases, and each database can contain\nmultiple queues.\n\n`liteq` uses SQLite to store a database of queues, and uses other,\ntemporary SQLites databases for locking, and finding crashed workers\n(see below).\n\n## Usage\n\n### Basic usage\n\n\n```r\nlibrary(liteq)\n```\n\nIn the following we create a queue in a temporary queue database.\nThe database will be removed if the R session quits.\n\n\n```r\ndb <- tempfile()\nq <- ensure_queue(\"jobs\", db = db)\nq\n```\n\n```\n#> liteq queue 'jobs'\n```\n\n```r\nlist_queues(db)\n```\n\n```\n#> [[1]]\n#> liteq queue 'jobs'\n```\n\nNote that `ensure_queue()` is idempotent, if you call it again on the same\ndatabase, it will return the queue that was created previously. So it is\nsafe to call it multiple times, even from multiple processes. In case of\nmultiple processes, the locking mechanism eliminates race conditions.\n\nTo publish a message in the queue, call `publish()` on the queue object:\n\n\n```r\npublish(q, title = \"First message\", message = \"Hello world!\")\npublish(q, title = \"Second message\", message = \"Hello again!\")\nlist_messages(q)\n```\n\n```\n#> id title status\n#> 1 1 First message READY\n#> 2 2 Second message READY\n```\n\nA `liteq` message has a title, which is a string scalar, and the message\nbody itself is a sting scalar as well. To use more complex data types in\nmessages, you need to serialize them using the `serialize()` function (set\n`ascii` to `TRUE`!), or convert them to JSON with the `jsonlite` package.\n\nTwo functions are available to consume a message from a queue.\n`try_consume()` returns immediately, either with a message (`liteq_message`\nobject), or `NULL` if the queue is empty. The `consume()` function blocks\nif the queue is empty, and waits until a message appears in it.\n\n\n```r\nmsg <- try_consume(q)\nmsg\n```\n\n```\n#> liteq message from queue 'jobs':\n#> First message (12 B)\n```\n\nThe title and the message body are available as fields of the message\nobject:\n\n\n```r\nmsg$title\n```\n\n```\n#> [1] \"First message\"\n```\n\n```r\nmsg$message\n```\n\n```\n#> [1] \"Hello world!\"\n```\n\nWhen a consumer is done processing a message it must call `ack()` on the\nmessage object, to notify the queue that it is safe to remove the message.\nIf the consumer fails to process a message, it can call `nack()` (negative\nackowledgement) on the message object. Then the status of the message will\nbe set to `\"FAILED\"`. Failed messages can be removed from the queue, or\nput back in the queue again, depending on the application.\n\n\n```r\nack(msg)\nlist_messages(q)\n```\n\n```\n#> id title status\n#> 1 2 Second message READY\n```\n\n```r\nmsg2 <- try_consume(q)\nnack(msg2)\nlist_messages(q)\n```\n\n```\n#> id title status\n#> 1 2 Second message FAILED\n```\n\nThe queue is empty now, so `try_consume()` returns `NULL`:\n\n\n```r\ntry_consume(q)\n```\n\n```\n#> NULL\n```\n\n### Crashed workers\n\nIf a worker crashes without calling either `ack()` or `nack()` on a message,\nthen this messages will be put back in the queue the next time a message is\nrequested from the queue.\n\nTo make this possible, each delivered message keeps an open connection to\na lock file, and crashed workers are found by the absense of this open\nconnection. In R basically means that the worker is considered as crashed\nif the R process has no reference to the message object.\n\nNote, that this also means that having many workers at the same time means\nthat it is possible to reach the maximum number of open connections by\nR or the operating system.\n\n## License\n\nMIT © Gábor Csárdi","jobInfo":{"package":"liteq","version":"1.0.1","parsingStatus":"success","parserVersion":1,"parsedAt":"2019-03-08T15:30:01+0000"}}

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