The Data Processing Library allows you to work with multiple versions of the same catalog so that you can, for example, compare and process changes between those versions. Besides the version of the catalog specified in the pipeline job configuration, a compiler can access the versions of the same catalog used in previous runs of the pipeline. More precisely, it can access all those versions that were used to compile earlier versions of the same output catalog.
Catalog Versions in the Data Processing Library
The job version of an input catalog is the version of the catalog provided to a batch pipeline through the pipeline job configuration. A catalog ID mentioned in the job configuration always refers to the job version of the catalog. There is no guarantee that a pipeline processes every version of an input catalog at least once or at most once. This is shown in the following diagram, where versions 1, 3, 4, and 6 of an input catalog are skipped, while version 2 is processed twice by the pipeline - which can happen if the pipeline is triggered manually, or another input catalog of the pipeline changed.
The Data Processing Library provides access to the versions of an input catalog used in previous successful runs of the same pipeline. These versions can be accessed through a PreviousRunId. For example, Default.PreviousRunId(inputA) derives a PreviousRunId from the catalog ID inputA mentioned in the job configuration, referencing the version of inputA used in the previous run. The following diagram shows which version the PreviousRunId refers to:
If the referenced version does not exist because the pipeline has not run before, the PreviousRunId references an empty catalog instead. In the example above, for the first pipeline job the PreviousRunId references an empty catalog, because the pipeline runs for the first time.
The diagram below illustrates that if a previous run fails, it is ignored. You can only use a previous run if it resulted in a successful publication of an output catalog version.
The examples above show only a single input catalog and a PreviousRunId going back exactly one run. Naturally, the concepts shown here extend to any number of input catalogs and going back any number of runs. For example, Default.PreviousRunId(inputA, 2) refers to the version of inputA processed two runs before the current run; the default is 1. Consequently, for the first two runs an empty catalog will be accessed through this catalog ID.
Similarly, you can also use this functionality to access previous versions of the output catalog. For example, the catalog ID Default.PreviousRunId(Default.OutCatalogId) refers to the output catalog produced by the previous successful run of the batch pipeline. A shorthand for this catalog ID is the feedback catalog ID Default.FeedbackCatalogId used for stateful processing.
NOTE Similar to stateful processing, all compilation patterns in the processing library still apply when you access a previous version of an input or output catalog. Moreover, this feature does not impact any concept, functionality, or require any special configuration in the environment where the application runs, typically the HERE platform.
Use CatalogViews to access a catalog in a specific version. A CatalogView corresponds to a restricted variant of the Catalog interface through which one version of the catalog can be accessed. The DriverContext can be used to acquire a CatalogView for any input catalog ID, including PreviousRunIds.
NOTE You can acquire Retrievers for a PreviousRunId through the inRetrievers method of the DriverContext.
Example: Comparing Catalog Versions
This example shows you how you can build a compiler that computes the difference between two versions of an input catalog using the Data Processing Library's ability to access multiple versions of the same catalog.
The topology-geometry layer in the HERE Map Content catalog contains, among others, road topology segments with unique IDs, as well as their geometry in the form of coordinate sequences. This example loads two different versions of the layer and outputs a JSON document containing all IDs of segments that were added, removed, or modified between these two versions.
Configuring Input Layers
To process a previous version in a compiler, use a PreviousRunId in the inLayers configuration, as shown in the following snippet:
We want to compare partitions with the same partition name, so we need to assign those partitions the same output key in the compileIn phase of the compilation. Since each input partition is mapped to exactly one output partition based on its partition name, we can employ a DirectMToNCompiler (see for more information). The mappingFn is implemented by simply replacing the catalog ID in the input key to construct the correct output key; the compileIn function is the identity function.
In the compileOut phase of the compilation, we use the retrievers to load both versions of the partition. The Key and Meta for both these partitions can be found in the intermediate data for the corresponding OutKey. In the following snippet, we load a map from segment IDs to Segment objects for both the previous and the latest version of the partition, using an empty map if one of the partitions does not exist. Then, we derive the sets of segments added, removed, or modified.
A complete version of this example is included in the examples/data-processing/scala/heremapcontent-difftool directory of the SDK.
Processing Fixed Versions of a Catalog
The Data Processing Library also provides FixedVersionIds, which always refer to the same version of a given catalog, independently of the version provided in the pipeline job configuration in current or in previous runs. FixedVersionIds can be used, for example, to associate data from the current version of the catalog with a fixed previous version of the catalog. For example, Default.FixedVersionId(inputA, 4) refers to version 4 of catalog ID inputA. Of course, the version passed to Default.FixedVersionId does not have to be a constant, but can instead be a value that was read, for example, from another input catalog or a configuration file.
NOTE If two subsequent runs of the same compiler use different fixed versions of a catalog, for example, because the versions are read from a configuration file, in the second run incremental compilation will be disabled. The Data Processing Library automatically ensures this by registering a Fingerprint of the set of all fixed versions used for a catalog. If the set of fixed versions remains the same between two runs, but the way the versions are used in the compiler logic changes, incremental compilation will not be disabled automatically, because the Fingerprints remain the same. In such cases, make sure to manually add a Fingerprint of whichever external source triggered the change in processing logic.