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-## 10. GTFS-realtime Extensions
-
-*Introduction to gtfs-realtime.proto* showed you an extract
-from the `gtfs-realtime.proto` file that is used as an input to the
-Protocol Buffers `protoc` command.
-
-One of the lines in this extract included the following `extensions`
-directive:
-
-```
-extensions 1000 to 1999;
-```
-
-Each element in a Protocol Buffers entity must be assigned a value. This
-is the value used to represent the element in the binary protocol buffer
-stream (for instance, the `trip` element was assigned a value of
-`1`). The `extensions` directive reserves the values between 1000
-and 1999 for external use.
-
-Having these values reserved means that transit agencies are free to
-include additional information in their own GTFS-realtime feeds. In this
-instance, the agency provides its own `proto` file to use with
-`protoc` that builds on the original `gtfs-realtime.proto` file.
-
-At the time of writing, the following extensions are defined:
-
-| Extension ID | Developer |
-| :----------- | :-------- |
-| 1000         | OneBusAway |
-| 1001         | New York City MTA |
-| 1002         | Google |
-| 1003         | OVapi |
-| 1004         | Metra |
-
-You can find this list at
-<https://developers.google.com/transit/gtfs-realtime/changes>). As more
-agencies release GTFS-realtime feeds this list will grow, since many
-agencies have specific requirements in the way their internal systems
-work, as well as to facilitate providing data in a way that is
-understood by users of the given transport system.
-
-To demonstrate how extensions are specified and used, the following case
-study will show you the extension for the New York City subway system.
-
-### Case Study: New York City Subway
-
-One of the agencies that provides an extension is New York City MTA.
-They add a number of custom fields to their subway GTFS-realtime feeds.
-
-The following is a slimmed-down version of their Protocol Buffers
-definition file, available from
-<http://datamine.mta.info/sites/all/files/pdfs/nyct-subway.proto.txt>.
-
-```
-option java_package = "com.google.transit.realtime";
-
-import "gtfs-realtime.proto";
-
-message TripReplacementPeriod {
-  optional string route_id = 1;
-  optional transit_realtime.TimeRange replacement_period = 2;
-}
-
-message NyctFeedHeader {
-  required string nyct_subway_version = 1;
-  repeated TripReplacementPeriod trip_replacement_period = 2;
-}
-
-extend transit_realtime.FeedHeader {
-  optional NyctFeedHeader nyct_feed_header = 1001;
-}
-
-message NyctTripDescriptor {
-  optional string train_id = 1;
-  optional bool is_assigned = 2;
-
-  enum Direction {
-    NORTH = 1;
-    EAST = 2;
-    SOUTH = 3;
-    WEST = 4;
-  }
-
-  optional Direction direction = 3;
-}
-
-extend transit_realtime.TripDescriptor {
-  optional NyctTripDescriptor nyct_trip_descriptor = 1001;
-}
-
-// NYCT Subway extensions for the stop time update
-
-message NyctStopTimeUpdate {
-  optional string scheduled_track = 1;
-  optional string actual_track = 2;
-}
-
-extend transit_realtime.TripUpdate.StopTimeUpdate {
-  optional NyctStopTimeUpdate nyct_stop_time_update = 1001;
-}
-```
-
-This file begins by importing the original `gtfs-realtime.proto` file
-that was examined in *Introduction to gtfs-realtime.proto*.
-
-It then defines a number of new element types (they choose to prefix
-them using `Nyct`, although the only important thing here is that they
-don't conflict with the names from `gtfs-realtime.proto`).
-
-This definition file then extends the `TripDescriptor` and
-`StopTimeUpdate` element types. Values
-`1000` to `1999` are reserved for custom extensions. This is the
-reason why the added `nyct_trip_descriptor` and
-`nyct_stop_time_update` fields use numbers in this range.
-
-Compiling an Extended Protocol Buffer
-
-In order to make use of these extended fields, you first need to
-download and compile the `nyct-subway.proto` file into the same
-directory as `gtfs-realtime.proto` (*Compiling gtfs-realtime.proto*).
-
-```
-$ cd /path/to/protobuf
-
-$ curl \
-  http://datamine.mta.info/sites/all/files/pdfs/nyct-subway.proto.txt \
-  -o nyct-subway.proto
-```
-
-You can now build a Java class similar to before, but using the
-`nyct-subway.proto` file as the input instead of
-`gtfs-realtime.proto`:
-
-```
-$ protoc \
-  --proto_path=/path/to/protobuf \
-  --java_out=/path/to/gtfsrt/src \
-  /path/to/protobuf/nyct-subway.proto
-```
-
-If this command executes successfully, you will now have a file called
-`NyctSubway.java` in the **./src/com/google/transit/realtime**
-directory (in addition to `GtfsRealtime.java`, which is still
-required).
-
-The next section will show you how to use this class.
-
-### Registering Extensions
-
-The process to load an extended GTFS-realtime is the same as a regular
-feed (in that you call **parseFrom()** to build the `FeedMessage`
-object), but first you must register the extensions.
-
-In Java, this is achieved using the `ExtensionRegistry` class and the
-**registerAllExtensions()** helper method.
-
-```
-import com.google.protobuf.ExtensionRegistry;
-
-...
-
-ExtensionRegistry registry = ExtensionRegistry.newInstance();
-
-NyctSubway.registerAllExtensions(registry);
-```
-
-The `ExtensionRegistry` object is then passed to **parseFrom()** as
-the second argument. The following code shows how to open and read the
-main New York City subway feed.
-
-***Note:** A developer API key is required to access the MTA
-GTFS-realtime feeds. You can register for a key at
-[http://datamine.mta.info](http://datamine.mta.info/), then substitute
-it into the key variable.*
-
-```java
-public class YourClass {
-  public void loadFeed() throws IOException {
-
-    String key = "*YOUR_KEY*";
-
-    URL url = new URL("http://datamine.mta.info/mta_esi.php?feed_id=1&key=" + key);
-
-    InputStream is = url.openStream();
-
-    ExtensionRegistry registry = ExtensionRegistry.newInstance();
-
-    NyctSubway.registerAllExtensions(registry);
-
-    FeedMessage fm = FeedMessage.parseFrom(is, registry);
-
-    is.close();
-
-    // ...
-
-  }
-}
-```
-
-Once the feed has been parsed, you will no longer need to pass the
-extension registry around.
-
-### Accessing Extended Protocol Buffer Elements
-
-Earlier it was explained that the general paradigm with reading
-GTFS-realtime data is to check the existence of a field using
-`hasFieldName()`, and to retrieve the value using
-`getFieldName()`.
-
-This also applies to retrieving extended elements, but instead of there
-being built-in methods for each extended field type, use
-`hasExtension(fieldName)` and `getExtension(fieldName)`.
-
-The argument passed to `hasExtension()` and `getExtension()` is a
-unique identifier for that field. The identifier is a static property of
-the `NyctSubway` class. For example, the `nyct_trip_descriptor`
-extended field has a unique identifier of
-`NyctSubway.nyctTripDescriptor`.
-
-Since this field is an extension to the standard `TripDescriptor`
-field, you can check for its presence as follows:
-
-```java
-TripUpdate tripUpdate = entity.getTripUpdate();
-
-TripDescriptor td = tripUpdate.getTrip();
-
-if (td.hasExtension(NyctSubway.nyctTripDescriptor)) {
-  // ...
-}
-```
-
-The `NyctSubway.nyctTripDescriptor` identifier corresponds to a field
-of the created class `NyctTripDesciptor`, meaning that you can
-retrieve its value and assign it directly to the class type.
-
-```java
-NyctTripDescriptor nyctTd = td.getExtension(NyctSubway.nyctTripDescriptor);
-```
-You can now access the values directly from this instance of
-`NyctTripDescriptor`. For example, one of the added fields is
-direction, which indicates whether the general direction of the train is
-North, South, East or West. The following code shows how to use this
-value:
-
-```java
-if (nyctTd.hasDirection()) {
-  Direction direction = nyctTd.getDirection();
-
-  switch (direction.getNumber()) {
-  case Direction.NORTH_VALUE: // Northbound train
-    break;
-
-  case Direction.SOUTH_VALUE: // Southbound train
-    break;
-  
-  case Direction.EAST_VALUE: // Eastbound train
-    break;
-  
-  case Direction.WEST_VALUE: // Westbound train
-    break;
-
-  }
-}
-```
-
-Similarly, you can access other extended fields, either from the
-`NyctTripDescriptor` object, or from an instance of the
-`NyctStopTimeUpdate` extension.
-
-### GTFS-realtime Extension Complete Example
-
-Piecing together the snippets from this case study, the following code
-shows in context how you can access the extra fields such as the
-train's direction and identifier:
-
-```java
-public class NyctProcessor {
-
-  // Loads and processes the feed
-  public void process(String apiKey) throws IOException {
-
-    URL url = new URL("http://datamine.mta.info/mta_esi.php?feed_id=1&key=" + apiKey);
-
-    InputStream is = url.openStream();
-
-    // Register the NYC-specific extensions
-
-    ExtensionRegistry registry = ExtensionRegistry.newInstance();
-
-    NyctSubway.registerAllExtensions(registry);
-
-    FeedMessage fm = FeedMessage.parseFrom(is, registry);
-
-    // Loop over all entities
-
-    for (FeedEntity entity : fm.getEntityList()) {
-      // In this example only trip updates are processed
-
-      if (entity.hasTripUpdate()) {
-        processTripUpdate(entity.getTripUpdate());
-      }
-    }
-  }
-
-  // Used to process a single trip update
-  public void processTripUpdate(TripUpdate tripUpdate) {
-
-    if (tripUpdate.hasTrip()) {
-
-      TripDescriptor td = tripUpdate.getTrip();
-
-      // Check if the extended trip descriptor is available
-
-      if (td.hasExtension(NyctSubway.nyctTripDescriptor)) {
-        NyctTripDescriptor nyctTd = td.getExtension(NyctSubway.nyctTripDescriptor);
-        
-        processNyctTripDescriptor(nyctTd);
-      }
-    }
-  }
-
-  // Process a single extended trip descriptor
-  public void processNyctTripDescriptor(NyctTripDescriptor nyctTd) {
-
-    // If the train ID is specified, output it
-    if (nyctTd.hasTrainId()) {
-      String trainId = nyctTd.getTrainId();
-
-      System.out.println("Train ID: " + trainId);
-    }
-
-    // If the direction is specified, output it
-
-    if (nyctTd.hasDirection()) {
-      Direction direction = nyctTd.getDirection();
-
-      String directionLabel = null;
-
-      switch (direction.getNumber()) {
-      case Direction.NORTH_VALUE:
-        directionLabel = "North";
-        break;
-
-      case Direction.SOUTH_VALUE:
-        directionLabel = "South";
-        break;
-
-      case Direction.EAST_VALUE:
-        directionLabel = "East";
-        break;
-        
-      case Direction.WEST_VALUE:
-        directionLabel = "West";
-        break;
-
-      default:
-        directionLabel = "Unknown Value";
-      }
-
-      System.out.println("Direction: " + directionLabel);
-    }
-  }
-}
-```
-
-After you invoke the `process()` method, your output should be similar
-to the following:
-
-```
-Direction: North
-
-Train ID: 06 0139+ PEL/BBR
-```