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Shaping and Type Fusion

Shaping​

Data that originates from heterogeneous sources typically has inconsistent structure and is thus difficult to reason about or query. To unify disparate data sources, data is often cleaned up to fit into a well-defined set of schemas, which combines the data into a unified store like a data warehouse.

In Zed, this cleansing process is called "shaping" the data, and Zed leverages its rich, super-structured type system to perform core aspects of data transformation. In a data model with nesting and multiple scalar types (such as Zed or JSON), shaping includes converting the type of leaf fields, adding or removing fields to "fit" a given shape, and reordering fields.

While shaping remains an active area of development, the core functions in Zed that currently perform shaping are:

  • cast - coerce a value to a different type
  • crop - remove fields from a value that are missing in a specified type
  • fill - add null values for missing fields
  • order - reorder record fields
  • shape - apply cast, fill, and order

They all have the same signature, taking two parameters: the value to be transformed and a type value for the target type.

Another type of transformation that's needed for shaping is renaming fields, which is supported by the rename operator. Also, the yield operator is handy for simply emitting new, arbitrary record literals based on input values and mixing in these shaping functions in an embedded record literal. The fuse aggregate function is also useful for fusing values into a common schema, though a type is returned rather than values.

In the examples below, we will use the following named type connection that is stored in a file connection.zed and is included in the example Zed queries with the -I option of zq:

type socket = { addr:ip, port:port=uint16 }
type connection = {
    kind: string,
    client: socket,
    server: socket,
    vlan: uint16
}

We also use this sample JSON input in a file called sample.json:

{
  "kind": "dns",
  "server": {
    "addr": "10.0.0.100",
    "port": 53
  },
  "client": {
    "addr": "10.47.1.100",
    "port": 41772
  },
  "uid": "C2zK5f13SbCtKcyiW5"
}

Cast​

The cast function applies a cast operation to each leaf value that matches the field path in the specified type, e.g.,

zq -Z -I connection.zed "cast(this, <connection>)" sample.json

casts the address fields to type ip, the port fields to type port (which is a named type for type uint16) and the address port pairs to type socket without modifying the uid field or changing the order of the server and client fields:

{
    kind: "dns",
    server: {
        addr: 10.0.0.100,
        port: 53 (port=uint16)
    } (=socket),
    client: {
        addr: 10.47.1.100,
        port: 41772
    } (socket),
    uid: "C2zK5f13SbCtKcyiW5"
}

Crop​

Cropping is useful when you want records to "fit" a schema tightly, e.g.,

zq -Z -I connection.zed "crop(this, <connection>)" sample.json

removes the uid field since it is not in the connection type:

{
    kind: "dns",
    server: {
        addr: "10.0.0.100",
        port: 53
    },
    client: {
        addr: "10.47.1.100",
        port: 41772
    }
}

Fill​

Use fill when you want to fill out missing fields with nulls, e.g.,

zq -Z -I connection.zed "fill(this, <connection>)" sample.json

adds a null-valued vlan field since the input value is missing it and the connection type has it:

{
    kind: "dns",
    server: {
        addr: "10.0.0.100",
        port: 53
    },
    client: {
        addr: "10.47.1.100",
        port: 41772
    },
    uid: "C2zK5f13SbCtKcyiW5",
    vlan: null (uint16)
}

Order​

The order function changes the order of fields in its input to match the order in the specified type, as field order is significant in Zed records, e.g.,

zq -Z -I connection.zed "order(this, <connection>)" sample.json

reorders the client and server fields to match the input but does nothing about the uid field as it is not in the connection type:

{
    kind: "dns",
    client: {
        addr: "10.47.1.100",
        port: 41772
    },
    server: {
        addr: "10.0.0.100",
        port: 53
    },
    uid: "C2zK5f13SbCtKcyiW5"
}

As an alternative to the order function, record expressions can be used to reorder fields without specifying types. For example:

zq -Z 'yield {kind,client,server,...this}' sample.json

also produces

{
    kind: "dns",
    client: {
        addr: "10.47.1.100",
        port: 41772
    },
    server: {
        addr: "10.0.0.100",
        port: 53
    },
    uid: "C2zK5f13SbCtKcyiW5"
}

Shape​

The shape function brings everything together by applying cast, fill, and order all in one step, e.g.,

zq -Z -I connection.zed "shape(this, <connection>)" sample.json

reorders the client and server fields to match the input but does nothing about the uid field as it is not in the connection type:

{
    kind: "dns",
    client: {
        addr: 10.47.1.100,
        port: 41772 (port=uint16)
    } (=socket),
    server: {
        addr: 10.0.0.100,
        port: 53
    } (socket),
    vlan: null (uint16),
    uid: "C2zK5f13SbCtKcyiW5"
}

To get a tight shape of the target type, apply crop to the output of shape, e.g.,

zq -Z -I connection.zed "shape(this, <connection>) | crop(this, <connection>)" sample.json

drops the uid field after shaping:

{
    kind: "dns",
    client: {
        addr: 10.47.1.100,
        port: 41772 (port=uint16)
    } (=socket),
    server: {
        addr: 10.0.0.100,
        port: 53
    } (socket),
    vlan: null (uint16)
}

Error Handling​

A failure during shaping produces an error value in the problematic leaf field. For example, consider this alternate input data file malformed.json.

{
  "kind": "dns",
  "server": {
    "addr": "10.0.0.100",
    "port": 53
  },
  "client": {
    "addr": "39 Elm Street",
    "port": 41772
  },
  "vlan": "available"
}

When we apply our shaper via

zq -Z -I connection.zed "shape(this, <connection>)" malformed.json

we see two errors:

{
    kind: "dns",
    client: {
        addr: error({
            message: "cannot cast to ip",
            on: "39 Elm Street"
        }),
        port: 41772 (port=uint16)
    },
    server: {
        addr: 10.0.0.100,
        port: 53 (port)
    } (=socket),
    vlan: error({
        message: "cannot cast to uint16",
        on: "available"
    })
}

Since these error values are nested inside an otherwise healthy record, adding has_error(this) downstream in our Zed pipeline could help find or exclude such records. If the failure to shape any single field is considered severe enough to render the entire input record unhealthy, a conditional expression could be applied to wrap the input record as an error while including detail to debug the problem, e.g.,

zq -Z -I connection.zed '
  yield {original: this, shaped: shape(this, <connection>)}
  | yield has_error(shaped)
    ? error({msg: "shaper error (see inner errors for details)", original, shaped})
    : shaped
  ' malformed.json

which produces

error({
    msg: "shaper error (see inner errors for details)",
    original: {
        kind: "dns",
        server: {
            addr: "10.0.0.100",
            port: 53
        },
        client: {
            addr: "39 Elm Street",
            port: 41772
        },
        vlan: "available"
    },
    shaped: {
        kind: "dns",
        client: {
            addr: error({
                message: "cannot cast to ip",
                on: "39 Elm Street"
            }),
            port: 41772 (port=uint16)
        },
        server: {
            addr: 10.0.0.100,
            port: 53 (port)
        } (=socket),
        vlan: error({
            message: "cannot cast to uint16",
            on: "available"
        })
    }
}) (error({msg:string,original:{kind:string,server:{addr:string,port:int64},client:{addr:string,port:int64},vlan:string},shaped:{kind:string,client:{addr:error({message:string,on:string}),port:port=uint16},server:socket={addr:ip,port:port},vlan:error({message:string,on:string})}}))

If you require awareness about changes made by the shaping functions that aren't surfaced as errors, a similar wrapping approach can be used with a general check for equality. For example, to treat cropped fields as an error, we can execute

zq -Z -I connection.zed '
  yield {original: this, cropped: crop(this, <connection>)}
  | yield original==cropped
    ? original
    : error({msg: "data was cropped", original, cropped})
  ' sample.json

which produces

error({
    msg: "data was cropped",
    original: {
        kind: "dns",
        server: {
            addr: "10.0.0.100",
            port: 53
        },
        client: {
            addr: "10.47.1.100",
            port: 41772
        },
        uid: "C2zK5f13SbCtKcyiW5"
    },
    cropped: {
        kind: "dns",
        server: {
            addr: "10.0.0.100",
            port: 53
        },
        client: {
            addr: "10.47.1.100",
            port: 41772
        }
    }
})

Type Fusion​

Type fusion is another important building block of data shaping. Here, types are operated upon by fusing them together, where the result is a single fused type. Some systems call a related process "schema inference" where a set of values, typically JSON, is analyzed to determine a relational schema that all the data will fit into. However, this is just a special case of type fusion as fusion is fine-grained and based on Zed's type system rather than having the narrower goal of computing a schema for representations like relational tables, Parquet, Avro, etc.

Type fusion utilizes two key techniques.

The first technique is to simply combine types with a type union. For example, an int64 and a string can be merged into a common type of union (int64,string), e.g., the value sequence 1 "foo" can be fused into the single-type sequence:

1((int64,string))
"foo"((int64,string))

The second technique is to merge fields of records, analogous to a spread expression. Here, the value sequence {a:1}{b:"foo"} may be fused into the single-type sequence:

{a:1,b:null(string)}
{a:null(int64),b:"foo"}

Of course, these two techniques can be powerfully combined, e.g., where the value sequence {a:1}{a:"foo",b:2} may be fused into the single-type sequence:

{a:1((int64,string)),b:null(int64)}
{a:"foo"((int64,string)),b:2}

To perform fusion, Zed currently includes two key mechanisms (though this is an active area of development):

Fuse Operator​

The fuse operator reads all of its input, computes a fused type using the techniques above, and outputs the result, e.g.,

echo '{x:1} {y:"foo"} {x:2,y:"bar"}' | zq -z fuse -

produces

{x:1,y:null(string)}
{x:null(int64),y:"foo"}
{x:2,y:"bar"}

whereas

echo '{x:1} {x:"foo",y:"foo"}{x:2,y:"bar"}' | zq -z fuse -

requires a type union for field x and produces:

{x:1((int64,string)),y:null(string)}
{x:"foo"((int64,string)),y:"foo"}
{x:2((int64,string)),y:"bar"}

Fuse Aggregate Function​

The fuse aggregate function is most often useful during data exploration and discovery where you might interactively run queries to determine the shapes of some new or unknown input data and how those various shapes relate to one another.

For example, in the example sequence above, we can use the fuse aggregate function to determine the fused type rather than transforming the values, e.g.,

echo '{x:1} {x:"foo",y:"foo"} {x:2,y:"bar"}' | zq -z 'fuse(this)' -

results in

<{x:(int64,string),y:string}>

Since the fuse here is an aggregate function, it can also be used with group-by keys. Supposing we want to divide records into categories and fuse the records in each category, we can use a group-by. In this simple example, we will fuse records based on their number of fields using the len function:

echo '{x:1} {x:"foo",y:"foo"} {x:2,y:"bar"}' | zq -z 'fuse(this) by len(this) | sort len' -

which produces

{len:1,fuse:<{x:int64}>}
{len:2,fuse:<{x:(int64,string),y:string}>}

Now, we can turn around and write a "shaper" for data that has the patterns we "discovered" above, e.g., if this Zed source text is in shape.zed

switch len(this) (
    case 1 => pass
    case 2 => yield shape(this, <{x:(int64,string),y:string}>)
    default => yield error({kind:"unrecognized shape",value:this})
)

when we run

echo '{x:1} {x:"foo",y:"foo"} {x:2,y:"bar"} {a:1,b:2,c:3}' | zq -z -I shape.zed '| sort -r this' -

we get

error({kind:"unrecognized shape",value:{a:1,b:2,c:3}})
{x:"foo"((int64,string)),y:"foo"}
{x:2((int64,string)),y:"bar"}
{x:1}