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Persistent Queues with Tape Prateek Srivastava

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curl -X "POST" "https://api.segment.io/v1/track" \ -H "Content-Type: application/json" \ -u apiKey: \ -d "{\"event\":\"hello world\",\"userId\":\"prateek\"}"

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Background Tasks

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Networks ar flaky

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Battery life is limited

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RAM is volatile

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Guarantees

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Durability

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Atomicity

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Efficiency

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–Murphy’s Law “Anything that can go wrong, will go wrong.”

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Solutions

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java.io.File https://developer.android.com/reference/java/io/File.html

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AtomicFile https://developer.android.com/reference/android/util/AtomicFile.html https://developer.android.com/reference/android/support/v4/util/AtomicFile.html

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if (backupFile.exists()) { file.delete(); backupFile.renameTo(file); }

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SharedPreferences https://developer.android.com/reference/android/content/SharedPreferences.html

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SQLite https://developer.android.com/reference/android/database/sqlite/package- summary.html https://developer.android.com/training/basics/data-storage/databases.html

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Tape https://github.com/square/tape

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✔ fast ✔ durable ✔ atomic

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File System

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renaming is atomic

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File temp = new File("file.tmp"); initialize(temp); File file = new File(“file.real"); f.renameTo(file);

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fsync is durable

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segment writes are atomic

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RandomAccessFile

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class RandomAccessFile { void seek(long pos); read{}(); void write{}(T data); }

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new RandomAccessFile(file, "rwd");

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-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.

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Element 3 Element 2 Element 1 File Header

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@jake 16 16 1 4096 Last Element Position First Element Position Count File Size 8 bytes 8 bytes 4 bytes 63 bits

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@jake 16 16 1 4096 Last Element Position First Element Position Count File Size 8 bytes 8 bytes 4 bytes 63 bits

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Version 1 1 bit Version 0 1 bit @jake

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@1.2.3

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@1.2.3

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Adding Elements

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queueFile.add(data);

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int newLastPosition = lastPosition + 4 + lastLength; 4096 1 32 32 Element 1 10

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raf.seek(newLastPosition); raf.write(data.length); 4096 1 32 32 26 Element 1 10

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raf.seek(newLastPosition + 4); raf.write(data); 4096 1 32 32 Element 1 10 Element 2 26

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raf.seek(0); writeHeader(fileLength, elementCount + 1, firstPosition, newLastPosition); 4096 2 32 46 Element 1 10 Element 2 26

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4096 1 32 32 26 Element 1 10 4096 1 32 32 Element 1 10 Element 2 26 4096 1 32 32 Element 1 10

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Removing Elements

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queueFile.remove();

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int newFirstPosition = firstPosition + 4 + firstLength; 4096 2 32 46 Element 1 10 Element 2 26

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writeHeader(fileLength, elementCount - 1, newFirstPosition, lastPosition); 4096 1 46 46 Element 1 10 Element 2 26

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raf.seek(oldFirstPosition); byte[] empty = new byte[firstLength + 4]; raf.write(empty); 4096 1 46 46 Element 2 26

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4096 1 46 46 Element 1 10 Element 2 26 4096 1 46 46 Element 2 26

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API @master

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class QueueFile { void add(byte[] data); byte[] peek(); void remove(); int size(); } @1.2.3

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class QueueFile { void add(byte[] data); byte[] peek(); void remove(); int size(); } @master

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class QueueFile { void add(byte[] data); byte[] peek(); void remove(); int size(); } @master

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class QueueFile { void add(byte[] data); byte[] peek(); void remove(); int size(); } @master

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Usage

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// Read and remove the data at the // head of the queue. byte[] element = queueFile.remove(); // Do something with the data. process(element);

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// Read and remove the data at the // head of the queue. byte[] element = queueFile.remove(); // Do something with the data. process(element);

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// Read data at the head. byte[] element = queueFile.peek(); // Do something with the data. process(element); // Remove data at the head. queueFile.remove();

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Example

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byte[] message = queueFile.peek(); Connection connection = open(“/v1/track"); connection.getOutputStream().write(message); connection.close(); queueFile.remove();

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Handling HTTP Errors

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retry network errors

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retry server errors (5xx)

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ignore client errors (4xx)

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try { connection.close(); } catch (IOException e) { return; // Network error, retry later. } int responseCode = connection.getResponseCode(); if (responseCode >= 500) { return; // Server error, retry later. } // Client error, e.g. invalid JSON. Don’t retry. if (responseCode >= 400) { log("server rejected message"); } queueFile.remove();

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@master class QueueFile implements Iterable { void add(byte[] d, int offset, int count); boolean isEmpty(); void remove(int n); void clear(); }

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class QueueFile implements Iterable { void add(byte[] d, int offset, int count); boolean isEmpty(); void remove(int n); void clear(); } @master

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class QueueFile implements Iterable { void add(byte[] d, int offset, int count); boolean isEmpty(); void remove(int n); void clear(); } @master

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class QueueFile implements Iterable { void add(byte[] d, int offset, int count); boolean isEmpty(); void remove(int n); void clear(); } @master

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@master class QueueFile implements Iterable { void add(byte[] d, int offset, int count); boolean isEmpty(); void remove(int n); void clear(); }

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Usage

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Iterator iterator = queueFile.iterator(); while (iterator.hasNext()) { byte[] data = iterator.next(); process(data); iterator.remove(); }

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// Read the data at the head of the queue. List data = new ArrayList<>(); for (byte[] d : queueFile) { data.add(d); } // Do something with the elements. process(data); // Remove elements from the queue. queueFile.remove(data.size());

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Example

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HttpURLConnection connection = open(“/v1/import"); int count = 0; int size = 0; for (byte[] data : queueFile) { if (size + data.length > MAX_LIMIT) { break; } write(data, connection); count++; size += data.length; } connection.close(); queueFile.remove(count);

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ObjectQueue API @master

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@master abstract class ObjectQueue implements Iterable { abstract void add(T entry); abstract T peek(); List peek(int max); List asList(); void remove(); abstract void remove(int n); void clear(); abstract int size(); }

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@master abstract class ObjectQueue implements Iterable { abstract void add(T entry); abstract T peek(); List peek(int max); List asList(); void remove(); abstract void remove(int n); void clear(); abstract int size(); }

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@master abstract class ObjectQueue implements Iterable { abstract void add(T entry); abstract T peek(); List peek(int max); List asList(); void remove(); abstract void remove(int n); void clear(); abstract int size(); }

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@1.2.3 interface Converter { T from(byte[] bytes); void toStream(T o, OutputStream os); }

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ObjectQueue Converter QueueFile

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ObjectQueue queue = ObjectQueue.createInMemory(); ObjectQueue queue = ObjectQueue.create(file, converter);

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Usage

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// Read data at the head. Pojo pojo = queue.peek(); // Do something with the data. process(pojo); // Remove data at the head. queue.remove();

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// Read the data at the head of the queue. List data = new ArrayList<>(); for (Pojo pojo : queue) { data.add(pojo); } // Do something with the elements. process(data); // Remove elements from the queue. queue.remove(data.size());

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Beyond Tape

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GCM Network Manager https://developers.google.com/cloud-messaging/network-manager

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Firebase JobDispatcher https://github.com/firebase/firebase-jobdispatcher-android

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JobScheduler https://developer.android.com/reference/android/app/job/JobScheduler.html

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Android Job http://evernote.github.io/android-job/

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Priority JobQueue https://github.com/yigit/android-priority-jobqueue