Update documentation on I/O

README.md

@@ -1,25 +1,40 @@
 # k4FWCore (key4hep FrameWork Core)
 
-k4FWCore is a Gaudi package that provides the PodioDataService, which allows to
-use podio-based event data models like EDM4hep in Gaudi workflows.
+k4FWCore is a Gaudi package that provides the IOSvc, which allows to
+use EDM4hep in Gaudi workflows.
 
-k4FWCore also provides the `k4run` script used to run Gaudi steering files.
+k4FWCore also provides the `k4run` script used to run Gaudi steering files. See the [documentation](doc/k4run-args.md) for more information.
 
 ## Components
 
 ### Basic I/O
 
-#### k4DataSvc
+| Current | Legacy | Description |
+|---------|--------|-|
+| IOSvc | k4DataSvc | Service handling the PODIO types and collections |
+| Reader | PodioInput | Algorithm to read data from input files on disk. |
+| Writer | PodioOutput | Algorithm to write data to an output file on disk. |
+| MetadataSvc | MetaDataHandle | Service/Handle handling user defined metadata |
 
-Component wrapping the PodioDataService to handle PODIO types and collections.
+See the [documentation](doc/PodioInputOutput.md) for more information.
 
-#### PodioInput
+### Auxiliary
 
-Algorithm to read data from one or multiple input file(s) on disk.
+### Collection Merger
 
-#### PodioOutput
+Algorithm merging multiple collections of the same type into a single collection.
 
-Algorithm to write data to an output file on disk.
+### EventHeaderCreator
+
+Algorithm creating a new `edm4hep::EventHeaderCollection` data object.
+
+### EventCounter
+
+Algorithm counting processed events and printing heart-bit.
+
+### UniqueIDGenSvc
+
+Service generating unique, reproducible numbers to be used for seeding RNG used by the algorithms. See the [documentation](doc/uniqueIDGen.md) for more information.
 
 ## k4run
 ```
@@ -57,6 +72,8 @@ print(my_opts[0].foo)
 
 * Gaudi
 
+* EDM4HEP
+
 ## Installation and downstream usage.
 
 k4FWCore is a CMake project. After setting up the dependencies (use for example `source /cvmfs/sw.hsf.org/key4hep/setup.sh`)

doc/LegacyPodioInputOutput.md

@@ -0,0 +1,136 @@
+<!--
+Copyright (c) 2014-2024 Key4hep-Project.
+
+This file is part of Key4hep.
+See https://key4hep.github.io/key4hep-doc/ for further info.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+-->
+# Legacy reading and writing EDM4hep files in Gaudi with the 4DataSvc
+
+:::{caution}
+`k4DataSvc` is a legacy service previously used in K4FWCore for reading and writing data in EDM4hep or other data models based on PODIO.
+
+The currently used service is `IOSvc`, which offers improved streamlined functionality and better support for modern workflows. For detailed documentation on `IOSvc`, refer to [this documentation](PodioInputOutput.md).
+:::
+
+This page will describe the usage of legacy [k4FWCore](https://github.com/key4hep/k4FWCore)
+facilities to read and write EDM4hep. This page also assumes a certain
+familiarity with Gaudi, i.e. most of the snippets just show a minimal
+configuration part, and not a complete runnable example.
+
+## The `k4DataSvc`
+
+Whenever you want to work with EDM4hep in the Gaudi based framework of Key4hep,
+you will need to use the `k4DataSvc` as *EventDataSvc*. You can instantiate and
+configure this service like the following
+
+```python
+from Gaudi.Configuration import *
+from Configurables import k4DataSvc
+
+evtSvc = k4DataSvc("EventDataSvc")
+```
+
+**It is important that the name is `EventDataSvc` in this case, as otherwise
+this is an assumption from Gaudi.** Once you have the `k4DataSvc` instantiated,
+you still have to make the `ApplicationMgr` aware of it, by making sure that the
+`evtSvc` is in the list of the *external services* (`ExtSvc`):
+
+```python
+from Configurables import ApplicationMgr
+ApplicationMgr(
+    # other args
+    ExtSvc = [evtSvc]
+)
+```
+
+## Reading events
+
+To read events you will need to use the `PodioInput` algorithm in addition to
+the [`k4DataSvc`](#the-k4datasvc). Currently, you will need to pass the input
+file to the `k4DataSvc` via the `input` option but pass the collections that you
+want to read to the `PodioInput`. We are working on making this (discussion
+happens in this [issue](https://github.com/key4hep/k4FWCore/issues/105)). The
+parts of your options file related to reading EDM4hep files will look something
+like this
+
+```python
+from Configurables import PodioInput, k4DataSvc
+
+evtSvc = k4DataSvc("EventDataSvc")
+evtSvc.input = "/path/to/your/input-file.root"
+
+podioInput = PodioInput()
+```
+
+It is possible to change the input file from the command line via
+```bash
+k4run <your-options-file> --EventDataSvc.input=<input-file>
+```
+
+By default the `PodioInput` will read all collections that are available from
+the input file. It is possible to limit the collections that should become
+available via the `collections` option
+
+```python
+podioInput.collections = [
+  # List of collection names that should be made available
+]
+```
+
+## Writing events
+
+To write events you will need to use the `PodioOutput` algorithm in addition to
+the [`k4DataSvc`](#the-k4datasvc):
+
+```python
+from Configurables import PodioOutput
+
+podioOutput = PodioOutput("PodioOutput", filename="my_output.root")
+```
+
+By default this will write the complete event contents to the output file.
+
+### Writing only a subset of collections
+
+Sometimes it is desirable to limit the collections to a subset of all available
+collections from the EventStore. The `PodioOutput` allows to do this via the
+`outputCommands` option that takes a list of `keep` or `drop` commands. Each
+command must consist of the `keep`/`drop` command and a target. The target is a
+collection name that may include the `?` or `*` wildcard patterns. This might
+look like the following
+
+```python
+podioOutput.outputCommands = ["keep *"]
+```
+
+which will keep everything (the default), while
+
+```python
+podioOutput.outputCommands = ["drop *"]
+```
+
+will simply drop all collections and effectively write an empty file (apart from
+some metadata). A common pattern is to `"drop *"` and then selectively adding
+`keep` collections to keep, e.g. to only keep the highest level MC and reco
+information:
+
+```python
+podioOutput.outputCommands = [
+    "drop *",
+    "keep MCParticlesSkimmed",
+    "keep PandoraPFOs",
+    "keep RecoMCTruthLink",
+]
+```