Persistent Queues

Offloading tasks to the background is a great way to simulate the feeling of responsiveness. But a lot can go wrong. This talk takes a deep dive into the implementation of Tape, a library that lets your app reliably process tasks in the background. It explores the API, suggests patterns for common use cases, and suggest alternatives for when Tape's simplicity isn't enough.

Transcript

1. Persistent Queues
5. -Murphy’s Law “Anything that can go wrong, will go wrong.”
7. Networks are flaky
10. Batteries are limited
13. Memory is constrained
15. RAM is volatile
16. Tasks must be durable
17. Non-Volatile Storage
18. Files java.io.File AtomicFile
19. java.io.File lacks atomicity
20. AtomicFile burdens caller with recovery
21. if (backupFile.exists()) { file.delete(); backupFile.renameTo(file); }
22. SharedPreferences loads entire queue into memory swallows errors
23. SQLite complex
24. Tape https://github.com/square/tape
25. QueueFile https://www.parleys.com/tutorial/ 5148922b0364bc17fc56c97b/chapter15
26. atomic durable efficient
27. What do we start with?
28. File System renaming is atomic fsync is durable segment writes are atomic
29. class RandomAccessFile { void seek(long pos); <T> read{<T>}(); void write{<T>}(T data); }
30. new RandomAccessFile(file, "rwd");
31. “rwd” Open for reading and writing, as with "rw", and also require that every update to the file's content be written synchronously to the underlying storage device.
32. Putting it all together
36. Adding
37. queueFile.add(data); *not to scale
38. int next = lastPosition + 4 + lastLength;
39. raf.seek(next); raf.write(data.length);
40. raf.seek(next + 4); raf.write(data);
41. raf.seek(0); writeHeader(fileLength, elementCount + 1, firstPosition, newLastPosition);
42. = =
43. Removing
44. queueFile.remove();
45. int newFirstPosition = firstPosition + 4 + firstLength;
46. writeHeader(fileLength, elementCount - 1, newFirstPosition, lastPosition);
47. raf.seek(firstPosition); byte[] empty = new byte[firstLength + 4]; raf.write(empty);
48. =
49. API
50. class QueueFile { void add(byte[] data); byte[] peek(); void remove(); void clear(); int size(); void forEach(ElementReader reader); } @1.2.3
51. interface ElementReader { void read(InputStream in, int length); }
52. // Read the data at the head of the queue. byte[] element = queueFile.peek(); // Process it. process(element); // Remove if from the queue. queueFile.remove();
53. example byte[] message = queueFile.peek(); if (message != null) { return; } HttpURLConnection connection = open("https://segment.com/import"); connection.getOutputStream().write(message);
54. Handling Retries
55. retry server and network errors
56. ignore client errors
57. try { connection.close(); } catch (IOException e) { // Network error, retry later. return; } int responseCode = connection.getResponseCode(); if (responseCode >= 500) { // Server error, retry later. return; } // Client error, e.g. invalid json. Don’t retry. if (responseCode >= 400) { log("server rejected message"); } queueFile.remove(); example
58. Idempotent Tasks
59. UUID.randomUUID().toString()
60. Batch
61. example if (queueFile.size() < 0) { return; } HttpURLConnection connection = open("https://segment.com/import"); OutputStream out = connection.getOutputStream(); queueFile.forEach((in, length) -> { Utils.copy(in, out); });
62. example for (int i = 0; i < n; i++) { queueFile.remove(); }
63. Enhanced Batch API
64. class QueueFile { void remove(int n); int forEach(ElementVisitor visitor); } @master
65. interface ElementVisitor { boolean read(InputStream in, int length); }
66. interface ElementVisitor { boolean read(InputStream in, int length); }
67. List<byte[]> elements = read(queueFile); process(elements); queueFile.remove(elements.size());
68. example HttpURLConnection connection = open("https://segment.com/import"); OutputStream out = connection.getOutputStream(); *int count = 0; // simplified queueFile.forEach((in, length) -> { if (count + length > MAX_LIMIT) { return; } count += Utils.copy(in, out); return; });
69. example HttpURLConnection connection = open("https://segment.com/import"); OutputStream out = connection.getOutputStream(); *int count = 0; // simplified queueFile.forEach((in, length) -> { if (count + length > MAX_LIMIT) { return false; } count += Utils.copy(in, out); return true; });
70. example HttpURLConnection connection = open("https://segment.com/import"); OutputStream out = connection.getOutputStream(); *int count = 0; // simplified queueFile.forEach((in, length) -> { if (count + length > MAX_LIMIT) { return false; } count += Utils.copy(in, out); return true; });
71. Queue<T> API
72. @1.2.3 interface ObjectQueue<T> { void add(T entry); T peek(); void remove(); int size(); void setListener(Listener<T> listener); }
73. interface Listener<T> { void onAdd(ObjectQueue<T> queue, T entry); void onRemove(ObjectQueue<T> queue); }
74. @master class FileObjectQueue<T> { List<T> peek(int n); List<T> asList(); void remove(int n); }
75. @1.2.3 interface Converter<T> { T from(byte[] bytes); void toStream(T o, OutputStream os); }
76. FileObjectQueue Converter QueueFile
77. Beyond Tape
78. backoff priority *dependencies *battery, network
79. Priority JobQueue https://github.com/path/android-priority-jobqueue
80. JobScheduler https://developer.android.com/reference/android/app/job/ JobScheduler.html
81. GCM Network Manager https://developers.google.com/cloud-messaging/network- manager
82. Android Job https://github.com/evernote/android-job
83. Cassette* https://github.com/segmentio/cassette
84. Questions? @f2prateek f2prateek.com