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Adding a new Shelley-based era to consensus

This document walks through the steps required to add a new Shelley-based era to consensus and to add it as an extra era to the Cardano blockchain.

Prior art upon which this is based:

The steps are fairly straightforward. We have put things in place (in consensus and in the ledger) when adding the Allegra and Mary eras that should now make it much easier to add a new additional era.

There are two main driving changes: adding the new era to Ouroboros.Consensus.Shelley.Eras and including that era in Ouroboros.Consensus.Cardano.Block. The compiler should point the rest of the way. It’s mostly a matter of following the existing patterns by imitating the previous Shelley-based case. Be sure to run both the Shelley (ouroboros-consensus-shelley-test) and the Cardano (ouroboros-consensus-cardano-test) tests.

For exhaustiveness, we give an overview of the changes needed. The new era we’ll be adding is the Alonzo era, which comes after the Mary era.


  • Locate the new tag in the ledger, e.g., AlonzoEra. This is an empty data type that is used at the type level to indicate the era. The ledger should have an instance of the ShelleyBasedEra class (the class defined in Shelley.Spec.Ledger.API, do not confuse it with the class with the same name in consensus) for this era. This class should provide all the instances consensus integration will rely on.

  • Note that both the affected consensus libraries and test suites are parametric in the Shelley-based era or in the general era. The test suite relies on the necessary Arbitrary, Show, Eq, … instances to be available for all of the Cardano eras. This means that a new era can only be added to the Cardano eras when the ledger provides all the necessary instances for the new era, including the instances for the test suite (e.g., the serialisation generators). Note that because the test suite is parametric/generic in the eras, it is not easy to temporarily omit a single era from the test suite until the required instances have been added. This means that we can’t extend the library with the new era without extending the test suite too. In the past, this has been the main blocker: the library was ready, but the required instances in the ledger were missing for our test suite(s).

  • Add the required dependencies on the ledger package (and its test package) to the cabal.project file. You will have to add it to some other cabal files too, let the compiler tell you which ones.


  • Define StandardAlonzo in Ouroboros.Consensus.Shelley.Eras and add any missing instances to that module, update the export list appropriately.

  • In Ouroboros.Consensus.Shelley.Node, define ProtocolParamsAlonzo just like the existing ones. In case the era in question needs extra parameters (e.g., a new genesis config?), they can be added here.


  • In Test.Consensus.Shelley.Examples, define an examplesAlonzo similar to how examplesMary is defined. If necessary, i.e., when new type families have been created in the ledger (e.g., TxOut), the examples function might have to take more arguments. This is where the golden test examples are defined, but only the Shelley ones are tested as part of this test suite. The others are only tested as part of the ouroboros-consensus-cardano-test test suite.


  • In Ouroboros.Consensus.Cardano.Block, include AlonzoEra in CardanoEras and ShelleyBasedEras. Update all the pattern synonyms in the module with the new era. Don’t forget to update the comments, the COMPLETE pragmas, and the export lists. It’s easy to forget a case and the compiler will likely not warn you, you’ll notice it when trying to use the pattern synonyms.

  • In Ouroboros.Consensus.Cardano.CanHardFork, update CardanoHardForkConstraints, add additional translations to the CanHardFork instance.

  • In Ouroboros.Consensus.Cardano.Node, update the SerialiseHFC instance by following the existing patterns. Add a new CardanoNodeToNodeVersion and CardanoNodeToClientVersion that enable the AlonzoEra, update the existing ones so that they disable the new era. Be sure to include the new versions in the two methods of the SupportedNetworkProtocolVersion instance. Extend protocolInfoCardano with the new era by following the type errors and adding the missing parameters (including ProtocolParamsTransition). Don’t forget to derive maxMajorProtVer from the new final era. Update protocolClientInfoCardano too.

  • In Ouroboros.Consensus.Cardano, update the ProtocolCardano type synonym, add the extra arguments needed for protocolInfoCardano to the ProtocolCardano constructor. Update protocolInfo accordingly.


  • In Test.Consensus.Cardano.Generators, update arbitraryNodeToNode, arbitraryNodeToClient. Try to understand the logic of these two, you will also have to add a new case to these. It would be nice if we could write these two in a generic way so that they won’t have to be updated with each era, but it’s not so simple. Update the other functions/instances. Be careful, you won’t get warnings for missing cases in Arbitrary instances, so go over all of them and add the missing cases.

  • In Test.Consensus.Cardano.ByronCompatibility, update toCardanoCodecConfig.

  • In Test.Consensus.Cardano.Serialisation, update testCodecCfg and dictNestedHdr.

  • In Test.Consensus.Cardano.Examples, update eraExamples, combineEras, and the rest.

  • In Test.Consensus.Cardano.Golden, update the ToGoldenDirectory instances.

  • Create a Test.ThreadNet.MaryAlonzo module similar to Test.ThreadNet.AllegraMary, to test the transition from Mary to Alonzo. Don’t forget to include it in the Main of the test suite. You will have to create a Test.ThreadNet.TxGen.Alonzo module similar to Test.ThreadNet.TxGen.Mary.

  • Run the golden tests of ouroboros-consensus-cardano-test. Golden test results should have been created for the new Cardano versions. Don’t forget to commit those files, otherwise they will be recreated on each run in CI and not compared against the previous results, rendering them useless.

  • Extend Test.ThreadNet.Cardano with the new era. At the time of writing this hasn’t been done yet for Allegra or Mary, though.