OverviewProjectsRecordsTagsStorageNamingQueriesIndexingDiffsTrashConcurrency ControlPasswordsBackupsProj Meta


Folio is the tag database used by to store and organize your data. The tagging model allows you to easily design free-form, dynamic models of your data. But it also provides first class support for the Haysatck ontology so that you can model your data in a standardized and reusable manner.

Folio organizes data into a three level hierarchy:

  • Projects: or "projs" are the top level unit of organization used to group records together (typically corresponds to a real-life project). Projects encapsulate a flat list of records, there are no pre-defined tree structures or tables in Folio.
  • Records: or "recs" are the basic unit of data modeling. Records are essentially associative arrays defined by a flat map of tags
  • Tags: tags are the leaf level of the model. A tag is a name/value pair


In Haxall a node hosts exactly one project (we just call it the database). SkySpark servers may host multiple projects. Projects are used to group records together into a single "database". The following features operate at the project level:

  • Database security (although the user database is shared between projects)
  • Backup
  • Queries and filter pathing

Projects must be named using a legal programmatic name and must be four chars or longer. Projects of three letters or less are reserved, as well as some special names. To take advantage of cloud/clustering support, it is highly recommended to use a unique project name for all your projects.

Projects are physically stored on the file system and structured as follows:

// haxall hosts one database under the "db" directory
  db/          // database files
  io/          // Axon I/O scratch directory
  backup/      // backup zip files

// skyspark hosts multiple databases under the "proj" directory
        db/          // database files
        io/          // Axon I/O scratch directory
        backup/      // backup zip files

In SkySpark the project name is strictly defined by the the directory name under the "var/proj/" directory. While the system is shutdown this directory can be renamed or copied to a new name.


A record or rec is the basic unit of modeling in the Folio database. All records are a Haystack dict - a map of name/value pairs. The tags assigned to a record are free-form; you may add, update, or remove tags at anytime. For IoT data you should model your dicts according to the standardized ontology.

There are a couple tags which have special meaning:

  • id: all records have a required id tag with a Ref value which uniquely identifies the record
  • mod: a DateTime timestamp indicating the last time the record was modified; this value is used for concurrency control
  • dis: display name is an optional tag which should be used on any record that models data an end-user would see; this is the default tag used as the "title" of the record and links to the record; also see Etc.dictToDis


Tags are the name/value pairs stored in a record. The name of a tag must follow the standard rules for Naming. The value of the tag is any of the Haystack kinds: one of the following scalar types:

  • Marker: indicates the tag is used solely to mark the record; markers are typically used to assign the record into a "type"
  • Ref: identifier for Folio records
  • Bool: true of false
  • Number: 64-bit floating point number with optional Units
  • Str: unicode string
  • Uri: universal resource identifier per RFC 3986
  • Date: standard Fantom date class
  • Time: standard Fantom hour of day class
  • DateTime: standard Fantom timestamp with timezone
  • Bin: binary streamed data stored on file system
  • Coord: geographic coordinate in latitude/longitude
  • XStr: extended type string
  • List: linear sequence of zero or more items
  • Dict: nested dict of name/value pairs
  • Grid: nested two dimensional grid

Note that although Bool is supported, convention is to use presence of a marker tag.


Folio persists data to disk, but operates as an in-memory based database. Records are read from disk on startup and stored in RAM for fast access. This design supports the real-time nature of sensor data. But it also imposes limits on Folio since most hardware tend to have less RAM than disk space.


Programmatic names are used by Folio for features such as:

  • as tag names
  • as Grid column names
  • as project names
  • as value of the name tag

Programmatic names use camelCaseNaming as follows:

  • first char must be ASCII lower case letter: a - z
  • rest of chars must be ASCII letter or digit: a - z, A - Z, 0 - 9, or _


The APIs for querying a Folio database are based on filters. Filters allow you to construct predicates using basic boolean logic and comparison operators. Filters support pathing: any tag with a Ref value may be traversed using the -> operator during the query operation.

The following types of queries are supported:

  • readAll(): query all the records which match a filter
  • read(): query a single record which matches a filter
  • readById(): optimized lookup by id


All queries to a Folio project take the form of a predicate Filters which is used to match a set of records. In the simplest case, each record in a project is scanned and checked against the filter for a match. Because the records are stored in RAM this operation is very fast; a run-of-the-mill server and can roughly scan 10K records every millisecond. This time will scale up linearly with your database.

The following sections describe the query optimization in SkySpark. Query optimization is not available in Haxall.

Query Optimizer

To optimize performance as the number of records grow, Folio will build and maintain a memory based index for "hot" tags. The query optimizer uses these indexes to avoid scanning every record. Lets take an example query:

site and equipRef==xxxxx

This query has two tags which are used: site and equipRef. In the case of site we only care if a rec has the tag (we don't care about its value). In the case of equipRef we care if a rec has the tag and it has a specific Ref value. Folio indexing handles both cases: it indexes which records have a tag, but it also sub-indexes the tag values.

The query optimizer will select the best index to use for the scan. If we have 300 recs with the tag site (whatever the value might be) and we have 15 recs with the tag/value pair equipRef==xxxx, then the query optimizer will chose the smallest index. In this case it will chose the equipRef==xxxx bucket and we only have to scan 15 items before determining the result.

Auto Indexing

You do not need to configure anything special to use indexing. Folio always keeps track of what tags are being used by queries. As soon as it detects a "hot" tag, it will automatically build an index for that tag. The current algorithm indexes a tag once it has been used in 100 queries.

Query Tuning

A full scan of a large database with millions of recs might take 10s of milliseconds. This might be fast enough for populating user interface screens, but when distributed across multiple functions it can add up quickly. So if working with large databases it is important to ensure that hot queries are utilizing the index.

The best way to analyze queries is with the Debug | Folio view, which provides a wealth of statistics on tag indexes and the queries run.

The Reads by Tag section lists statistics on all tags which have been analyzed for optimization:

  • tag: name of the tag used by a query filter
  • numReads: number of times this tag has been used in a query where the tag was eligible for index optimization
  • indexed: if indexed, the number of recs which have this tag

The Reads by Plan lists statistics on all the query plans chosen by the query optimizer:

  • plan: type of plan and tag name
  • numReads: number of reads against using the plan
  • totalTime: total time running numReads with the plabn
  • avgTime: average time it takes to run this query plan

Note that the current query optimizer cannot use the index for NOT filters and OR filters. Examples:

ahu and not rooftop        // can use ahu index, but not rooftop index
ahu or chiller             // will be unoptimized
equip and (ahu or chiller) // will use equip index


Modifications to a Folio database are encapsulated as diffs. Diffs are a set of changes to apply to a record. Diffs work just like a patch file in a version control system. Diffs include the ability:

  • add or remove a record
  • update/add a set of tags on an existing record
  • remove a set of tags on an existing record (using the special remove value)

In Fantom diffs are committed with the Folio.commit method:

db.commit(Diff(rec, Etc.makeDict1("change", "new-val")))

In Axon diffs are committed using the diff() and commit() function:

commit(diff(rec, {change:"new-val"}))

Transient Diffs

In general when a diff is committed, it is written to the file system for durability. However, if your application has rapidly changing real-time data this can cause serious performance issues. To support real-time data, Folio supports the concepts of transient diffs. Transient diffs are applied only to the in-memory representation of the records, but are not serialized to the file system. Transient diffs do not update the mod tag of the record.


Records are moved into the trash bin by adding the trash marker tag. Trash recs continue to operate in the database just like any other record except they are not included in any filtered queries unless an explicit {trash} option is used. Note that funcs that work with id refs like readById() and readLink() will return trashed recs. Also, Haystack filters that filter only by id (e.g. id == @ref) are optimized to use readById() and will also return trashed recs.

Concurrency Control

All records support the required mod tag indicating the timestamp of their last persistent modification. This timestamp is used to implement optimistic concurrency control. This model allows queries and diffs to operate without explicit locking. When constructing diffs, they are passed the version of the rec which was read. If during the diff commit the database detects that the record has been modified since the last read, then the commit fails with a ConcurrentChangeErr.

Diffs support the ability to force a commit to by-pass concurrency control. This is typically used when updating status tags under complete control of a given application. Transient diffs to not update the mod tag, however unless the the force flag is used they are still checked for concurrent change.


There are two types of passwords stored in a project:

  • one-way: user accounts store a salted one-way hash of password typically using a SCRAM cryptographic hash
  • two-way: many connectors require passwords to establish their connections. These passwords must be two-way in that they must be written, but also read back in plaintext form

In both cases, the hash or the password is a secret to be protected. The hash is safer in that if compromised it is difficult to compute the original password. These secrets are stored outside of the core folio database in a file called "passwords.props". Pulling them out of the folio database makes them easier to secure.

The basic philosophy used to protect the passwords file is that only write access is granted external to the VM process (via Axon and REST API). Reads of the passwords must be done within process. External apps may store passwords using the passwordSet() function. Internal code only may access the password database using Folio.passwords method.


Folio supports the ability to take a backup of a project during runtime. A backup is a zip file which includes an atomic copy of the records, tags, and history data.

Proj Meta

Every folio project should have exactly one rec with the projMeta tag. This record is used to store project wide settings. The following tags may be configured on the projMeta rec:

  • dis: display name for the project if you wish to use a string other than the project's programmatic name
  • doc: summary string for the project
  • steadyState: delay for steady state transition

In addition to the tags above the system automatically maintains a version tag on the projMeta record (do not modify this tag).