As stated before, datums can be located on points, axes,edges, and surfaces. Here is how different types of features are called out on engineering drawings. The symbol must be placed on or with one single extension line out of the datum surface. This means that the datum is only on the surface of the part where it is shown. However if the part is round like a cylinder, the datum would be the entire surface of the cylinder.
For Axis control, the symbol must be placed on the dimension of a diametric tolerance as shown above. This means that the datum is actually the central axis through the feature, and not the feature surface itself. A datum feature is usually an important functional surface where all other dimensions need to be in spec with reference to it. However, things get a little tricky when clarifying what the datum actually is.
A Datum is a perfect point, line, plane or surface but only exists theoretically. However a Datum Feature is a tangible surface, point or axis on a part where that theoretical datum is located. It will always have some kind of waviness, bumps and valleys. To form this theoretical plane in the real world, measurement surfaces or points are used. For example,the face of a flat part may be referenced by a datum on a drawing. In the above drawing, this surface may be simulated by a near perfect granite slab. They are usually important functional surface.
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Actually, I was having an argument with my engineer about the checking method for parallelism, my method is by lay down the datum on granite table then used indicator to move along the parallel controlled surface area. My engineer mentioned, is the correct method is by using micrometer or caliper which is for me for that is more to thickness checking. That was the first case. The idea follows the model developed by the Puppet, Chef and Ansible communities possibly others , in the configuration data management area:.
Although not in v1 yet, Datum is currently used in Production to manage several hundreds of machines, and is actively maintained. A stable v1 release is expected for March , while some concepts are thought through, and prototype code refactored.
To simplify the key concept, a Datum hierarchy is some blocks of data nested hashtables organised in layers, so that a subset of data can be overridden by another block of data from another layer. You can transform the Data by overriding what you want in the per Node override :.
The order of precedence you define for your layers define the Most specific at the top of your list , to the Most generic at the bottom. On the file system, this data could be represented in two folders, one per layer, and a Datum configuration file, a Datum. This demonstrate the override principle, but it will always return the same thing. How do we make it relative to a Node's meta data? The idea is that we want to apply the override only on certain conditions, that could be expressed like:. Let's create SRV02 for witness, which is in Paris the override won't apply.
And we configure the Datum. The overall goal, better covered in the book Infrastructure As Code by Kief Morris, is to enable a team to " quickly, easily, and confidently adapt their infrastructure to meet the changing needs of their organization ". To do so, we define our Infrastructure in a set of Policies: human-readable documents describing the intended result or, Desired State , in structured, declarative aggregation of data, that are also usable by computers: The Configuration Data. Finally, the decentralised execution of the platform can let the nodes converge towards their policy.
The policies and their execution are composed in layers of abstraction, so that people with different responsibilities, specialisations and accountabilities have access to the right amount of data in the layer they operate for their task. The abstraction via roles allows to apply a generic 'template' to all nodes, while enabling Node specific data such as Name, GUID, Encryption Certificate Thumbprint for credentials. At a high level, we can compose a Role that will apply to a set of nodes, with what we'd like to see configured. In this document, we define a generic role we intend to use for Windows Servers, and include the different Configurations we need Shared1,SoftwareBaseline.
The Software baseline for this role is self documenting. Its specific data apply to that role, and can be different for another role, while the underlying code would not change. Adding a new package to the list is simple and does not require any DSC or Chocolatey knowledge. We define the nodes with the least amount of uniqueness, to avoid snowflakes. Below, we only say where the Node is located, what role is associated to it, its name SRV01, the file's BaseName and a unique identifier.
This is where the Configuration Data is massaged in usable ways for the underlying technologies DSC resources. We import the Module or DSC Resources needed by the Configurations, and for each Node, we lookup the Configurations implemented by the policies Lookup 'Configurations' , and for each of those we lookup for the parameters that applies and splat them to the DSC Resources sort of Although Datum has been primarily targeted at DSC Configuration Data, it can be used in other contexts where the hierarchical model and lookup makes sense.
The Datum hierarchy, similar to Puppet's Hiera , is defined typically in a Datum. This returns a hashtable with a key 'AllNodes' StoreName , by using the internal command under the hood :. Should you create a module e. You can have several root branches , of different Datum Store Providers , with custom options but prefer to Keep it super simple.
These are still very useful and used as an intermediary abstraction, such as the FileSystem provider used in the Datum FileProvider.
In short, I wanted an uniform key, that could abstract the container, storage, and the structure within the Format. Imagine the standard FileSystem provider:. PSD1 is in the folder. From the example above where we loaded our Datum Tree, we'd use the following to return the value:.
So we're just accessing variable properties, and our Config Data stored on the FileSystem, is just mounted in a variable in case of the FileProvider. This is why we have Typically, I mount the following structure with many more files not listed here :. But to be a hierarchy, there should be an order of precedence, and the lookup is a function that resolves a relative path , in the paths defined by the order of precedence.
Although you can configure Datum to behave differently based on your needs, like merging together the data found at each layer, the most common and simple case, is when you only want the 'MostSpecific' data defined in the hierarchy and this is the default behaviour. In that case, even if you usually define the data in the roles layer the most generic layer , if there's an override in a more specific layer, it will be used instead.
But the ordering shown above is not very flexible.
What is a datum?
How do we apply the relation between the list of roles, the current Node, Environment, location and so on? As we've seen that a Node implements a role , is in a location and from a specific Environment , how do we express these relations or any relation that would make sense in your context? We can define the names and values of those information in the Node meta data SRV And use variable substitution in the ResolutionPrecedence block of the Datum. Regardless of the Datum Store Provider used there's only the Datum File Provider built-in, but you can write your own , Datum tries to handle the data similarly to an ordered case-insensitive Dictionary, where possible i.
PSD1 don't support Ordering.
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All data is referenced under one variable, so it looks like a big tree with many branches and leafs like the one below. In the Tree described above, the Lookup function iterates through the ResolutionPrecedence's key prefix, and append the provided key suffix:. By default the merge behaviour is to not merge , which means the first occurence will be returned and the lookup stopped.
Datum identifies 4 main types in whatever matches first the following:. Their merge behaviour can be defined in the Datum. There is a default Behaviour MostSpecific by default , and you can specify ordered overrides:. This is the recommended setting and also the default, so that any sub-key merge has to be explicitly declared like so:. The key to be used here is the suffix as used in with the Lookup function: e.